It's worth saying that whatever they call it, GTA: Online is basically an MMO and it's one of the top three most popular game on Earth iirc. In fact I think only Tetris and Minecraft beat it.

Looking at what GTA6 is shaping up to be, and wondering how R* is evolving the online side, I think we might simply have seen MMO's turn into something people our age don't recognize.

30-player lobbies isn't exactly "massively" multiplayer though.