GTA ONLINE runs on a Cloud Server...Meaning our progress we do now can transfer to various platforms, including next gen platforms.
More than likely there will be PS4/Xbox One GTA V re-release, shortly after the pc version most likely.
I imagine once GTA V is released to every platform (360, PS3, PC, Xbox One, PS4) and has saturated it's potential player base, they will release GTA Online as a Free To Play title that will be a Stand Alone game that you can download off of PSN, Xbox Live, and Steam.
So why do I think there won't be a GTA 6 in the end of it all?
1. GTA V had a budget that reached 260 million, only behind Pirates of The Carribean (300 Million) as the most costly entertainment budget ever in the entertainment medium (films, shows, books, games, etc). The transition to Next Gen Consoles, along with loftier player "expectations" will no doubt inflate the budget once again, and could potentially bring it near 300 Million.
The problem with that though is that you are taking a gamble. Higher production costs must equate to higher returns. No, GTA isn't a brand that's stagnating, but still one could question if this obstacle (of inflated future gta iteration budgets) is something of a concern for Rockstar and it's Publisher Take-2.
2. Creating these titles takes Rockstar on Average 5 years to develop, and the work that comes along with recreating a whole new city from scratch, while developing new characters and narratives is something that is a big undertaking.
So here's ultimately why GTA Online might be the end to a potential GTA 6.
1. Adding additional content will be much quicker and speedier due to the Cloud Servers.
2. Rockstar discussed wanting to add Liberty City as a playable world in the future, meaning this could open possible new cities ..and when adding new cities what stops from Rockstar simply adding a new city with some new missions rather than a whole new 300 million stand alone release.
3. Microtransactions. It's much more cheaper for Rockstar to add new content and cities via the cloud server rather than make a whole new standalone release. Plus, online allows for microtransactions, which by providing constant feed of new items to purchase, you will constantly be getting profits for simply changing a line of code for 19.99.
I just think that in the end we will have a free to play GTA Universe, with various cities, and content to purchase...