I think the brand "GTA" has somehow become mainstream, palatable entertainment. The most average people (not as an insult, just an observation) are going to buy it as soon as possible, and if the promotional media is any indication, the main story's progression will probably be the requisite milquetoast 'slop' (as you so astutely called it) that wont offend that consumer. Perhaps I'm crazy wrong, too. The best games needn't kowtow to the whims of fad and trend, but at some point the AAA PR team got more control over game development than the visionaries.
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From a certain perspective, GTA was always "Mainstream": GTA III sold 14+ million copies; San Andreas 27. At the time, people would constantly refer to the fact that you could pick up hookers in the game and then drop them off and run them over (despite this being .1% of the gameplay). To some extent, the early GTA games even reached outside the ordinary "gamer" sphere, in the same way Tetris did, since the PS2 had a HUGE install base and the game was pretty simple to pick up and just start messing around with.
On the other hand, the point of the games was always to be a bit biting and sardonic. III couldn't afford a lot of licensed music, so they made a LOT of the tracks themselves, and one is literally a protest against the attempted censorship and rounding off of all the corners in culture that was happening at the time (and has only accelerated since):
"You're sweet, you're shy, you must admit
You censor your life for your benefit
But you lose all the taste, and you sit there and waste
Your day, your month, your year"
Notably, it's also a protest against stagnation and being locked into the past:
"Nothing stays, they pass you by
If you don't change, if you don't change"
And fair enough, I can see the irony. But not all change is good. Sometimes, a seed becomes a might oak. Other times a house, abandoned, rots to ruin. Or is vandalized.
Regardless, I don't think it's a dramatic contrarian position to claim that the entire ethos of the GTA games was to be a dramatic contrarian, almost to the point of self-parody at times. That this too has been ruined by the usual forces isn't surprising, but it is sad. The game just became too big, and at a certain size cool people with cool ideas can't retain control, and every decision is made by the board room and the HR department.