GTA Online is Rockstar's most successful addition to the GTA formula, so it would likely be an excellent fixture for the series from an economic standpoint.
It would be a bizarre business decision not to have something like GTA Online for future games. It makes a ludicrous amount of money off of Shark Cards, which allows the whole game to still be F2P. While players can spend real-life money to get ahead, the fact that players can still grind makes GTA Online appealing to a lot of people.
However, there is a discussion to be had on GTA Online's future. When GTA 6 is released, fans aren't quite sure what will happen to GTA Online. There will come a time when GTA 5, and by association, GTA 6, won't be the hottest GTA game around. Hence, it would be best if GTA 6 had its own online game that carried over a lot of the progress a player made in the prior GTA game.
Should GTA Online become a permanent fixture for the GTA series?
There are many ways to look at GTA Online's future. First, a person has to acknowledge that the game is highly successful. That is an objective claim, especially since players can see its massive playerbase on various platforms like Steamcharts. Likewise, various analysts also tend to see the economic upside to having a game like GTA Online feature microtransactions.
Second, there are the more subjective points of discussion. Subjectively, some players might be inclined to believe that GTA Online is one of the best games out there. Some players will strongly disagree with that statement, but most game critics (such as the ones found on Metacritic) would at least agree that GTA Online is a good game.
GTA Online's future
GTA Online, in its current form, has lasted longer than most other online games. Most online games are lucky to last a year or two, but the fact that GTA Online has been around since 2013 is crazy to think about. The GTA franchise is a highly successful video game series, so it isn't too ludicrous to think about. However, it's important to think about what can happen to GTA Online when the next big GTA game is released.
Since 2013, there has been no new GTA game. For comparison's sake, in the same time span of nearly eight years, fans got GTA III, GTA Vice City, GTA San Andreas, GTA Advance, GTA Liberty City Stories, GTA Vice City Stories, GTA 4 and its expansion packs, and GTA Chinatown Wars. It's understandable to see why some fans are frustrated with the lack of GTA 6.
However, there will come a point when GTA 6 is released. GTA 6 will be GTA 5's biggest competitor in terms of players, so GTA Online will likely lose some of its playerbase when this happens. However, GTA Online can still become a permanent fixture, just in a different way than what some fans might be expecting.
Why GTA Online should be a permanent fixture to the series
GTA Online doesn't have to be limited to GTA 5. Take Overwatch 2, for example. All of the player's progress from Overwatch 1 will be transferred to Overwatch 2. This would be an excellent way to preserve GTA Online's legacy. Imagine if players could transfer their money, vehicles, weapons, etc. to GTA 6's online equivalent. Then, when the time comes in another decade or two, another transfer can happen.
This way, Rockstar can still make significant profits. GTA 6 can take place in a new location, so not everything can be transferred in this scenario. Plus, Overwatch 1 is going to be cross-compatible with Overwatch 2, so GTA Online can still exist alongside GTA 6's version.
The main reason to keep GTA Online alive in some capacity is so Rockstar can earn a lot of money. That's understandable, but there are alternative ways to do it for both new creative ventures and to reinspire people's wallets compared to the current holding pattern.
Future GTA Online incarnations
The GTA series is popular enough that it can realistically have two or three online games without having a dead playerbase. Dividing it up might not be ideal, but when the alternative is killing off a game, there's not much that can be done.
Either GTA Online exists forever, and GTA 6 is never released, or GTA 6 is eventually released, and GTA Online's playerbase drops. Hence, it's best to move on. From an economic standpoint, plenty of mobile games shut down their original game and move on to another project to earn more money.
The main difference would be that GTA Online would still exist, just in a different form. However, it should always exist in some capacity for the sole fact that Rockstar can make millions off simple microtransactions, even if it's considered deplorable by most of the fanbase.