Rockstar Games still has a long future planned for GTA Online. At the very least, they intend for the game to remain popular and profitable through 2020, and probably longer. If a certain leak is to be believed, they also have a pretty extensive plan for upcoming DLC in store, including map expansions.

Rockstar Games also has Red Dead Redemption 2 in the pipeline, scheduled for an Autumn 2017 release. Naturally, their goal is for the game to do well in terms of sales, as well as remain popular following launch. Seeing as it will also feature an Online mode, likely with DLC and most possibly microtransactions, Rockstar will want it to achieve a similar level of success as GTA Online.
It's pretty obvious that GTA Online will bleed players when RDR2 launches, though it is likely that Rockstar isn't too worried about this effect since they won't lose any customers. However, the marketing emphasis placed on RDR2 will draw the mainstream spotlight from GTA, and this might end up hurting the latter.