The problem Rockstar is getting people to give up all the shit they've purchased in GTA5: Online and start from scratch, and you have to pay them $80 for the privilege (plus however much you've spent in GTA5, and likely more).
That's a tall order, Rockstar is banking on the power of the GTA brand to push through but there's a few issues with that.
One, the audience have grown older and may not have the leisure time to religiously play and paypig with Shark Cards, while the little shits that were meant to replace them are playing other stuff (Pedolox or Troonite).
The other issue is one of userbase. With GTA5 Rockstar was lucky enough to double-dip across two console generations, both having a healthy number of users, along with PC. GTA6 is launching exclusively on the PS5 to start with, and the PS5 is an already moribund platform - 50% of PS4 owners have yet to see a need to upgrade as an example, and the situation for both Sony and MS is so dire they're peddling their exclusives on any platform that wants to have them.
Not to say that GTA6 won't sell, because it's obvious it will sell a lot, but what the game needs to be is the single most profitable media product in the history of the human race, and I'm not sure it can do that.