You're looking it at it from the wrong perspective. The Inquisitor's story was very open before Trespasser - there was effectively no limit on what the Inquisitor could want or do at the end of DA:I. You had a superpowered being with an ancient elven reality-bending power, an organization that was said to be one of the most powerful in Thedas and didn't answer to anyone, and a potential threat that the Inquisitor could - or maybe wouldn't - get involved in any number of immeasurable ways.
I see what you mean, but if this is why Bioware did what they did, I disagree with the reasoning 100%. If they have to try this hard to wipe former PCs off the map (almost literally), they're constructing their stories in a convoluted way just so they can have new protagonists in each installment. Why not just continue for another game? Not forever, just another game? What would be so bad about that? If Bioware had never said "new game = new protagonist", would the same number of people complain about this?
Note that I'm not really asking you, just asking.
I guess I'm not the only one who felt "I lose" after Trespasser. It was even worse for me, for my main Inquisitor had gotten invested in some of the things she lost - her political power and most notably the anchor. Also, you had four opponents in Trespasser, and of those, you lose against three (Solas, Ferelden and Orlais) and win only against one (the qunari).
Plus, you really only win against the Qunari because of Solas. Dragon Age: Inquisition - Get Trolled and Used by Solas $14.99
Knowing Solas, he will free all elves in slavery and recruit them for his army, effectively ending most slavery in both Tevinter and Qunari lands.
This actually seems like a likely way to set up reforming Tevinter - having Solas remove most of the slaves and force them to rethink their ways. Not because they are convinced, but because they must.
Imagine how amazing it would be to play a character who is all but broken. It would be the perfect reason to play someone who thinks "damn the consequences, i've tried it your way before"
It would be interesting, in general. This is by no means a requirement, but good games sometimes make me feel that rage, the rage my protagonist would feel, and I direct it into the game (usually near the climax, during combat/roleplaying options). It's satisfying when a game makes you care that much and lets you show it.
That wasn't written as well as I'd like.