There is a huge degree of uncertainty over GTA Online's future when GTA 6 is released.
GTA 6 hasn't been announced yet, but some fans still wonder what will happen to GTA Online when it is released. It can take years for that to become a reality. GTA 6's base game will likely blow GTA 5 out of the water, but fans assume there will be a GTA Online equivalent in that game sometime after launch.
Considering how profitable GTA Online is, it's unquestionably an interesting discussion to have. There's no simple black and white answer where creating GTA 6 instantly kills GTA Online, as there are layers of discussion to go through first.
What does the future of GTA Online look like alongside GTA 6?
As it stands right now, GTA Online is perfectly healthy and is still receiving major updates. GTA Online is still one of the most played games in the present, so something significant would have to happen to alter that. Considering GTA Online is a unique game unlike most other games, there isn't much direct competition except for a possible GTA 6 announcement.
How sequels tend to affect their predecessors in GTA
It's no secret that GTA 6 is hyped up beyond belief. Fans are eagerly awaiting any news, no matter how minor it is. However, it is important to note how sequels tend to affect their predecessors in a historical context. Some examples will be related to GTA, but others will be from outside of the series.
First, one must preface that introducing a sequel to a beloved franchise doesn't immediately kill the game that precedes it. There will still be fans that go back to that game, no matter how many years pass. A prime example of this is GTA San Andreas, which is still beloved by a sizable community. There are often new mods and events in fan-made multiplayer mods, so it's still going strong.
However, the main thing to note in a situation like that is that those players tend to pay for nothing. It isn't like GTA Online where new updates cost Rockstar money, and that Shark Cards are often a solution to that problem. If GTA Online's playerbase falls to a small but sizable fanbase, there could be some trouble for the game as a whole.
The feeling of a "dead game" stings in the gaming community, so if GTA 6 was released and managed to take away GTA 5's fanbase, then there will be fewer GTA Online players by default. If GTA 6 introduced an online equivalent of GTA Online, then it's likely that GTA Online's future will be significantly harmed.
How sequels tend to affect their predecessors in other games
GTA fans are loyal, but there are notable examples of games outside of the series. An example would be when fans attach themselves to the sequel of a popular Call of Duty game, and how over time, a once-popular Call of Duty game is only a shell of itself in the online community. Some games are timeless, but most aren't.
Historically, online games do have an expiration date. For example, 343 Industries will shut down the legacy Halo servers by the end of 2021. These Halo games used to be the talk of the town when they first came out on the Xbox 360, but fans of those games will have to play them by other means.
While GTA Online could survive when GTA 6 is released, there's a strong possibility that it could be shut down for good in the future. It won't be immediate and it could take decades, but it's a thought that looms over the heads of many fans.
GTA Online's future when GTA 6 is released
The best way for GTA Online to survive when GTA 6 is released is for the two games to be different enough to warrant players jumping back and forth between the two games. If GTA 6 is 100% superior to GTA Online, then it is likely that GTA Online will die sooner than expected.
Likewise, if Rockstar puts all of their attention to GTA 6 and neglects GTA Online, then a similar fate will occur. At best, GTA Online could be in maintenance mode for fans to enjoy the game as it is. Fans of GTA Online on the PS3 and Xbox 360 know how this feels, as they never got the updates players get on the later consoles.
If the time comes when GTA Online won't receive updates anymore, it's more likely for Rockstar to leave it in maintenance mode than to kill it immediately. It would eventually get the ax, but it will allow players to enjoy the game for what it is. Take a game like Realm Royale, for example. It wasn't profitable, so Hi-Rez just left in a state where players can still play it, but there won't be any future updates.
If GTA 6 is different enough from GTA Online
In a situation where GTA 6 is radically different from GTA Online (and GTA 5 by association), then GTA Online could still have a lucrative future. It may receive fewer updates, but it won't be a "dead game." In this situation, GTA 6 will still be the more popular game, but GTA Online would be a good alternative (and likely the cheap alternative for players with less money to throw around).
If GTA 6 is a bad game
While Rockstar has never released a GTA game that was critically panned, one can never say never. It's improbable that this scenario would occur, but if it did happen, then GTA Online's popularity would be more or less the same. GTA's legacy could be tarnished a bit, but thankfully this is the least likely scenario.