If GTA goes all online then you can be sure that another studio will come and take its place in providing such a single player experience and story. Although, most likely Take Two will try keep a minimum single player part in just to avoid giving people the impression that this is a series with no single player at all. If you haven't noticed Call of Duty does this very thing where most of the latest entries only have had a minimum single player part of 2-5 hours of content. I think there might've even been CoD games completely without a single player campaign.
Anyways, this is how competition and the markets work, and if you think about it these games mostly came in and grabbed the attention of gamers away from other games that weren't as interesting. Driver, Medal of Honor, Unreal Tournament, we don't see much of those anymore.
Do most people care that The Elder Scrolls (which is basically just Skyrim at this point) replaced a bunch of other western RPG series like Might and Magic, Wizardy (mostly the 8th) etc? No, most people don't, because The Elder Scrolls was better at grabbing players' attentions and interests.
I don't think it was a coincidence that when Mario went all linear style and all collect-a-thons had stopped being made, that this genre started resurging, giving us Yooka-Laylee, Spyro Reignited, A Hat in Time and a return to collect-a-thon for Mario too in Odyssey.
Unless you are interested in a very niche game genre, and it would have to be a very rare one these days with how many games come out, then I wouldn't be afraid of that not getting supplied from some studio or another.