With reports of GTA 5 shipping 70 million copies, and earnings reports indicating a constant rise in player numbers, it seems like an impossible prospect that the player base of Online might ever decline. It very likely won't either for the next few years, at the very least.
However, Rockstar will need to stay on their toes if they want to keep those numbers trending upwards. Considering over 5 years passed between the announcement of GTA 5 and Red Dead Redemption 2, chances are another 5 will fly by before the next Rockstar project (which is undoubtedly under development as we speak) is announced and 6 years until it is released. Keep in mind that there is no actual guarantee that said project will be GTA 6.
Now, Rockstar's plans are to keep GTA profitable at least until 2020, but ideally beyond. Hypothetically, if Red Dead Online doesn't reach the same levels of popularity as GTA Online or even more so, Online will need to be kept strong until 2023. And realistically, no matter how much hype surrounds RDR2 and how popular it seems to be with the main gaming community, it won't have the same mainstream reach as GTA does.
As a three-year-old game, the fact that GTA Online is gaining popularity as opposed to winding down is something of a miracle in its own right, and while the formula which led to GTA 5's lasting success is a gift that keeps giving, Rockstar can only ride this wave for so long.
So what will happen when suddenly the sales numbers stoop? What happens when the game's age causes players to migrate to other games, be it Red Dead Online or some other, non-Rockstar title? Presumably Rockstar will have a tough enough time keeping a hold of the hardcore crowd with RDR 2's release nearing, but it will get harder in the future.
Short digression: you might be confused as to why we're often speaking of GTA 5 and RDR2 as rivals when they're both developed by the same company. GTA Online's unprecedented monetary success makes it the primary financial pillar of Take-Two Interactive, and this is mainly thanks to the game's mainstream popularity and the fact that it's available on 5 different platforms.
Red Dead Redemption 2 will launch on two platforms and while it's famous among gamers, it doesn't have much mainstream weight. In spite of the importance of mainstream popularity, GTA Online still thrives on the hardcore crowd primarily to keep the community aspect alive.
If the core gamers move over to RDR2, finish the single player, and instead of being hooked by the multiplayer move on to other games, Rockstar will lose customers. Hence, they will need to do something big with GTA Online around the same time as RDR2 is launched to ensure that those who leave Los Santos in favor of the Wild West return once they're done.
This major splash can't be made by a mere GTA Online DLC pack, or even with something as major as Heists was. Rockstar will need to do something truly major, something significant enough to be newsworthy and deserving of headlines right next to the release of RDR2. These are what we call "doomsday DLCs", content concepts big enough to prevent the "end" of Online.
