I selected "The whole question is wrong" because I don't think things ever declined. Unlike most others on the codex who feel there's been a longterm decline starting about 10 to 20 or so years ago, I think it's always been the same. Companies are always cashing in, trying to make a buck. They will throw their customers to the curb if it benefits them. There're a spectrum of games from the low end to the high end, in population terms. Games on the low end serve a niche crowd, while games on the high end serve the broadest audiences. A game isn't just niche or mainstream - it's a mix of things between the extremes on either end.
When games age, no matter their audience, they display common characteristics which resemble all this too. This throws people off, confusing them. Average codexers see a (aging) game getting easier and more casual. Most will like this, but some won't. Many on this forum relate "getting easier and more casual" to the decline. Most mainstream games also seek to be easy and more casual because that's the primary goal of the mainstream. Aging games are desperate. One of the things they do to delay death is to broaden their appeal, effectively casualizing themselves. In MMO's, mudflation can also cause this because low level players might desire the higher levels for a variety of reasons, but mudflation might act as a wall. As a result, MMO games respond by making levelling to the mid and upper range faster. In combination, aging games take on the character of a company chasing the mainstream, but central to this action is a desperate will to survive, NOT a true desire to be mainstream.
The gaming industry is maturing, but it's not fundamentally changing. Kickstarter, frequently hoorahed as the beginning of niche, is an expression of deepening muscle, but that's all. Niche games have always existed, despite some on here insisting before kickstarter all games were mainstream or at least mainstream wannabees. This is simple noit true, and I fail to understand how a gamer who lived through those years can believe there were no niche games before kickstarter. It's so untrue it's laughable.
Before kickstarter, niche games were known as simply being less popular, or otherwise owing themselves to an audience different than the one which is present today. In another thread I used BC3000 AD as an example of a niche game. It was relased in the 1990's to horrible reviews and forums filled with flamewars. By all accounts, it was a disaster, but it gained a sort of cult or niche status. Simulations of that time also were unpopular, and by virtue of this were also niche. Elite-like games similarly were no popular - this is why they we4r rare. Fallout 1/2, by comparison, were much more popular at the time, but compared to todays games, they're niche. I do think the character of the audience 10 years ago was different from what it's today. NAturally, this is what happens when the mainstream grows. The mainstream of yesterday is more niche than the one of today becaus the population is greater.
I think older games are niche today because they served a smaller crowd when they were current. They cannot serve the larger crowd today because they were not geared for it. But some older games are more niche than others.
And note there're also technical issues with playing old games. I, for example, find it hard to play games which do not use the mouse. Most gamers today expect 3d or voiceovers or good GUI's or high resolution or wide screen support. Most gamers at least expect a game to use the fullness of their PC, whether it be AI or graphics or something else. Older games will not use modern systems fully. Even though these technical issues demand a role in this discussion, they can alos be different from person to person. For example, I can probably play and enjoy a text game more than most gamers, even though I prefer the mouse.