You're missing the point. the focus for this game was different than the past ones. This wasn't a traditional GTA revenge story. If anything, the people we play as are the real focal point, it's those characters that matter, and their interactions between eachother. Everyone else is secondary. if there's an antagonist in this game it's Michael or Trevor.
Then why does ending c who almost everyone thinks its the only ending and where rockstar put most effort in revolves entirely around killing stretch haines and devin weston(the antagonists) instead of solving the Michael/trevor conflict?
But how would having Franklin (a gangsta coming late to the game) killing either Michael or Trevor have been any emotionally satisfying resolution to the Trevor / Michael conflict? Much better (the way a movie would do it) is for Michael and Trevor to have a big final, emotionally wrenching, multistaged battle between them.
I agree that (1) the story could have capitalized on lingering Michael / Trevor unresolved issues. But actually, after the final heist, that was supposed to satisfy Trevor so he did not feel the need to take revenge on Michael, the need for resolution was supposedly gone. The story could have continued with Trevor continuing to antagonize Merriweather, so that Michael, Franklin and Dave would have to bail him out sometimes, and then maybe mutually decide that Trevor has to go because he is destroying the status quo and their peaceful lives, at which point, Michael should have the primary responsibility to take out Trevor and he could get Franklin and maybe Dave and Lester to help in a final big boss battle to take out Trevor.
But it seems like Rockstar either got tired of writing story, or just ran out of budget for making boss-level missions, and they they used Devin Westin (who was just a rich version of an over-the-top California-ized Brucie) as a cheap way to close off the story with the three protags in Ending C. But as was said above, Devin Weston was not worth the time it took to create the cutscenes.
Just medicore antag writing, I have to say, in general agreement with the OP.