As opposed to what you may have derived from previous comments and statements, it looks like Take-Two doesn't intend to rely on — or even continue to support — GTA Online indefinitely (at least in its current form). While the game is generating immense amounts of profit, even through microtransactions alone, the company will eventually pull the plug.
Of course, if you think about it for just a little while, it all makes sense. GTA Online is only worth supporting while it has a big enough player-base. However, the game is over three years old and although it is still growing instead of shrinking, the playerbase will eventually take a turn and start to decrease.
During the recent Cowen and Company Technology, Media & Telecom Conference, Strauss Zelnick, the CEO of Take-Two Interactive Software has stated that GTA Online was never intended to be a permanent addition to their line-up.
We do expect GTA Online's results to moderate because October will be three years since we released it. Not only was it not our intention that GTA Online was permanent, but it's important that it not be permanent. We have to rest the franchise at some point.
While the GTA series as a whole does indeed constitute a "permanent" product of Take-Two, Online alone does not. This rather odd emphasis on a decline that has no sign of happening is a stark contrast to the recent announcements regarding how long-lived Take-Two intends Online to be.

The game is expected to be profitable and therefore supported with content until 2020 at the least, with high chances of that period increasing. Despite Zelnick's comment about expected drops due to the game's age, it was the staggered release schedule accommodating different platforms that pushed Online into a strange perpetual state of constant sales.
By releasing the three versions of the game over the course of three years, Rockstar constantly re-infused GTA Online with hype and popularity. Today, Online is still among the most played multiplayer games in the world.