You're pretty delusional, and I mean that not as an insult, if you think that they would add an online to those games, they obviously are workin on GTA 6 and that is really easy to understand, because they don't want their game to be a dissapointment like a few really awaited releases were, just look at Cyberpunk.
They made so much money from GTA 5 back when it came out, and the gaming industry has grown ever since then, so it would not make sense, because GTA 5 online is making them money, but overtime people will get tired of it, which they have already, some type of way.
Release of GTA 6 which is arguably the most waited game of all time, would break the internet, and make them billions in the first weeks of release, and more and more as the time goes on, with an added online mode to that game even more.
They obviously won't give up on one of their most prestigious game titles, and possibly the most known game franchise in the world. So there's that. San Andreas already has multiplayer, so maybe if that can be integrated to the remastered franchise, new servers will open up and all that.
Also you said that the graphics will be really good, but the leaks and rumors say otherwise, because of the specs that will be needed for the game to be run, are not high. The game is also on Unreal Engine 4. They want to keep the nostalgic feel, while fixing everything that was wrong with the games.
It's gonna look different, but obviously not Mafia Definitive Editions type of different