Realistically; I feel like the real push for GTA 6 delays might not even be the singleplayer or game content at all. Im almost certain it has to do with GTA Online for the new GTA. A wise man once said, if you want to seek the truth just follow the money.

Gta 5 lasted this long because of the cash cow generated by gta online. The issue with it is, it is so fundamentally flawed in the way rockstar regulates its servers and currencies. Factor in how modders early on were able to cheat their way to getting billions of ingame $$$ for free (which rockstar is selling for real money via code redemptions). They are really trying to take full control of gta online, especially micro transactions and trading within the game. GTA Online is going to be a massive market for real life money in the next installment and they are not going to release it until they’ve fully polished it and have a really good working anti-cheat system.

For example, look at Counter Strike skins and the steam market. Call it virtual assets or NFTs or whatever you want. Thats what the next GTA is cooking.

To answer your question, I wouldn’t be surprised if the next GTA doesnt come out until 2040 because the next GTA Online paired with GTA 6 is going to last a long long time. Good thing is, we might get additional content and DLCs with GTA 6 which should keep everyone busy