I don't see them completely abandoning single player campaigns. There's still a lot of money to be made there. If anything, a profitable online cash cow can help fund newer projects. Also, they have too many respected IPs to just let them go to waste. There's plenty of evidence that a new Red Dead is coming, and not everyone likes playing online games.
I don't think Rockstar will stop making campaigns either but I do think it's possible. Look at Valve... Who would have thought that they would stop making games like Half Life, Portal, and Left 4 Dead? Probably not many people, especially in Half Life's case. That is exactly what they have done because they are making so much money from Steam, Dota, CS: GO.
Rockstar is in a different situation because they aren't making 30% off every game sold on Steam but they are still making tons of money of GTA Online. You are right that not everyone likes online games but they don't need to make everyone happy to make a ton of money. Their main priority from a business standpoint is probably the people who continue to pour money into GTA Online years after its release.
Again, I think the next GTA game will probably have a campaign. I can just see how they could come to the conclusion that financially it makes more sense not to bother investing millions of dollars into making a AAA campaign. An updated version of GTA Online with no campaign would not be for me but I have no doubt that it would still sell millions of copies and make tons of money by selling currency.
Edit: The new Red Dead game has probably been in development for years. It was probably started before they saw what a big success GTA Online would be. I don't think we will really know for sure what they are doing next until GTA 6 is announced.
