... how would have "A completely new and unrelated Lvl 18 character wakes up in a mysterious cave?" been better?
By not being tied to the supremely shitty Original Campaign. Pay attention.
Which you aren't required to play.
Hell, I've recommended MoTB to dozens of people, always pointing out that they don't need to play the OC.
As for a relevant insertion - the PC was (again) chosen because of the silver shard of the Sword of Gith. A tenuous connection, sure, but it was there.
Very obviously, such a change would've had some (albeit minute) effects on the narrative. By no stretch of the imagination is the shard necessary for the overall plot, however; hell, you could've easily have crashed there out of sheer chance.
It adds another narrative layer for those of us who've
played suffered through the OC. For others, it makes almost no difference at all, but does imply the PC wasn't a blank slate before waking up in that cave.
Going by that logic, every RPG in which you do some quest outside of strictly following the main plot - aka: practically every one of them - is bad.
Not at all. There's a huge difference between doing things that makes sense for the character as he is in the process of doing other, more or less related things in the same vein, around the same area, and going to a weeks-long trip to another part of the country or continent while your sanity is (poorly) implied to be deteriorating and a menacing plot being carried out with some hint of urgency.
Like I said, many RPGs feature a plot that by all rights should be the sole priority of the player. Take Fallout 1 & 2, for example. You
should make a very linear beeline from city to city, following the clues, so that your people don't die of thirst/starve, yet the games actively encourage you to go off exploring in locations that have nothing to do with the main story.
Very, very few games have side-quests that are almost all in some way or another connected to the main story/crisis.
How great the issue is in this depends largely on how things are conveyed to the player, it's the old Oblivion vs. Morrowind issue. In Morrowind, you are encouraged to explore and your adventuring makes perfect sense in the given context, even as the main plot unfolds. In Oblivion and Skyrim, doing any of the side-quests comes across as completely fucking nuts considering that the world is literally fucking shattering while you're fetching some fucking apples or something.
Even in Morrowind, it was implied that the Ghostgate could fail at any moment, and that you
should hurry up.
Another example of this is actually Baldur's Gate vs. Baldur's Gate II. Baldur's Gate was far superior in this, whereas in Baldur's Gate II you run around half the region for months while you have absolutely no idea (in an in-character sense) what's up with Imoen, who you're implied to care for (to the point where it's mandatory to progress the plot). Delaying past the point of actually raising the money makes small sense.
Actually, the PC in BG2 had some choice in reasoning why they wanted to go to Spellhold: 1) Imoen 2) Getting revenge on Irenicus 3) Getting Irenicus to spill the beans on what he meant by the PC's untapped power.
White March is one of the worst offenders, though - you literally leave the entirety of the main plot behind for a completely unrelated issue which you have very little knowledge or motivation to care for to the point where you'd travel there, risking widespread destruction and the loss of your own sanity. White March would've made a lot more sense as a post-OC expansion in a narrative sense, but that would've created it's own set of issues in terms of various content (i.e. outcomes of end slides) and would've been much harder to implement. It's a fucking train wreck, honestly, and a lot would've have to be rewritten and integrated differently in order for it to make any sense whatsoever.
In PoE, you have very little motivation to do anything but hunt Thaos. Period. The only RPG that handled that "you have to do it NOW, hurry, there's no time!" shtick well was MoTB.
Is your primary issue with this that the White March is in a different region? Because it takes about two, maybe three days to travel there, which isn't significantly more than other areas in the base game.
