A brief tour of the Decline of proper CRPGs (non blob, non action, party based P&P simulators);
Start with Pool of Radiance (1988), for an example of a CRPG that inclined the genre and was largely untouched by Decline.
Next, play Curse of the Azure Bonds (1989), and notice how several interface issues from PoR were cleaned up, but also note that many ambitious design concepts from PoR have been simplified, streamlined and/or eliminated. For example, parlaying with monsters, finite numbers of wandering monsters, hiring NPCs from the training hall, animating party members as undead, interesting stuff to find on the overland map, dungeons that sort of make sense, etc.
Next, play Secret of the Silver Blades (1990). Some of the Decline that I attributed to Curse of the Azure Bonds may actually have occurred here. The world map has been largely eliminated/rendered irrelevant and you're mostly just fighting endless wandering monsters in a linear dungeon that no longer makes much sense at all. Somehow there are hordes of dragons and high level mages hanging out in this abandoned mine. It's worth noting however that this is a low point for the Gold Box games. They would never again be as ambitious as Pool of Radiance (1988), but they would bring back the world map and generally settle into a formula that offered solid gameplay with some P&P style adventure and more of an illusion of freedom than SotSB.
If you're taking the extended tour, it might be interesting to take a look at the Dragonlance games at this point, there was certainly a significant potential for decline as the plot was to some extent driven by NPCs from the official (bad) Dragonlance novels. Many people see them as part of the Decline. However, I would NOT consider the Dragonlance series to be true Decline, as there were a number of additional mechanics added to make the games more complex and interesting and there isn't really a significant change in the amount of freedom that is offered (not much). If Tanis Half Elven hadn't showed up to make you do something, some generic NPC/event would have.
Next, play Dark Sun: Shattered Lands (1993). Aside from the turn based combat, this game should immediately feel familiar to early 2000s CRPG players. Notice how in-level exploration has been streamlined/simplified/reduced to a shroud removing formality by the switch from first person dungeon exploration to zoomed out third person exploration. Exploration has changed from a game into a task. Notice how much more time you spend on reading text boxes and clicking on options from lists. Notice how much less time you spend on tactical combat and adventuring. Notice how the party has been reduced to four members. Notice how there is more diversity in character options and how more spells are modeled, but also notice how the tactical combat has not significantly improved. Notice how much easier the game is if you select certain types of characters.
Next, play Fallout (1997). Fallout technically belongs to its own short lived sub genre of games, but it is also highly relevant to any discussion of Decline. Take note of how much time you're now spending on reading text and selecting options from lists. Take note of how the turn based combat is so simplistic that it barely even qualifies as a game anymore. Realize that nothing of value was lost when they switched this series over to shitty FPS combat. Finally, give Ian a submachine gun and stand in front of him in a fight.
Next, play Baldur's Gate (1998). This game is highly relevant to any chronology of the Decline, but in and of itself, it is not a bad game. However, it is worth noting that the strategic ganeplay involved in party building has been replaced by simply picking from a modest sized list of recruitable characters. BG popularized real time combat in CRPGs, but since turn based combat in CRPGs hadn't significantly inclined since the very early 90s, the combat in BG isn't actually problematic. Recruitable party members will occasionally interrupt gameplay to make you read text and click an option from a list, but it isn't a significant distraction yet. Not everything is positive for this game though, it really drives home how tedious the shroud clearing task can become.
Next, play Baldur's Gate 2 (2000). In absolute terms, this is without a doubt the best CRPG ever created, but if you take the year of creation into account you can't help but feel disappointed at how little things have improved since 1989. Virtually all future decline can be traced back to this game, but in most areas it was a better game than Baldur's Gate (1998), even though you need to click through a lot more text. However, it is worth noting that party building has been simplified even further as there are not very many recruitable characters to pick from anymore. Recruitable characters are interrupting gameplay more often, but it still isn't particularly irritating. Romances appear; they will eventually transform this into a genre for perverts, but they're quite tame and minor here.
Next, play Pool of Radiance: Ruins of Myth Drannor (2001). Don't play for long, but get a feel for the combat in this game. Obviously, a huge step down from Baldur's Gate 2. But amazingly, the combat is not much more interesting than the combat from Curse of the Azure Bonds (1989) and it takes far longer to play. They seem to have forgotten how to design turn based games.
Next, play Knights of the Old Republic (2003). Notice that information is no longer presented to the player in a clear, 2D format, but is now obscured by messy 3D graphics. Party building has declined further, you now have only two recruitable characters as party members. But don't worry, they'll nag you quite a bit.
Next, play Temple of Elemental Evil (2003). Play through Hommlet only, you can stop after that. Sorry, that was cruel, I just wanted you to experience the worst level ever included in a CRPG, three hours of walking around, reading mundane text and clicking on options from lists. This game did finally improve turn based combat in CRPGs above 1989 standards, but was crippled by horrible level design, so it did not have any significant effect at halting or reversing the decline, aside from (probably) inspiring KoTC.
Next, play Knights of the Old Republic 2 (2004). Information is again obscured by messy 3D graphics, but this time combat has been made so incredibly easy that it is almost a formality. Many players report going through the entire game with no need to use any consumables and there is little need to pause to consider your tactics. While this game still has the mechanically robust d20 system behind it, it is on par with Fallout in terms of terrible combat gameplay, simply because the game is so easy. With combat and exploration providing negligible gameplay, most of the "game" is about reading text and clicking on options from lists. We've almost reached full decline here, we just need some romances to attract the perverts.
Next, play Knights of the Chalice (2007). Take note of the many, many ways in which turn based combat in CRPGs is finally improved from the standards of 1989. Take note of the fact that this game did not sell enough to warrant a sequel (Codexers complained about the graphics quite loudly) and that the developer is now doing a real time strategy game.
Finally, play Dragon Age and Dragon Age 2. Just be sure to pirate them.
So, a summary of the decline. Initially, exploration provided gameplay in the form of first person mazes. This was eventually replaced by the task of clearing all the shroud from the map. Eventually, this was replaced with third person 3d levels, but this didn't bring back significant amounts of gameplay to exploration. (Image of completely linear DA2 mini map goes here).
Initially, turn based combat in CRPGs was good. But then it failed to incline (in CRPGs) for like 14 years. In some significant ways it actually got worse. By the time turn based combat inclined again, it was too late.
Initially, real time with pause combat in CRPGs was not bad. But then it got bad.
Initially, building a party offered strategic gameplay. But then you just got to pick your party from an ever shrinking list of RPCs. Eventually these RPCs started trying to molest you.
Initially, gameplay was the focus of RPGs. But eventually, reading (or listening to) words and clicking options from lists became the focus of RPGs. That and molesting RPCs.