They can support it as long as they want. I don't play it anymore. But I put hundreds of hours into the game, especially in 2014 and 2015. In 2016 GTA Online went to sh*t imo. Moved on to other games. To this day I don't understand why GTAOnline became more popular over time. Imo now it's just an endless grind. When you're done grinding you buy the overpriced cars, houses, planes etc. and then wait for the next update. Once the next update drops you'll probably sell some old stuff to make room for new vehicles. And then start grinding again of course. Makes no sense to me but to each their own!
But back to OP's question... I guess they will continue with the updates as long as they bring in the shark card millions. I'm sure it will slow down a bit once Red Dead's out but like they already said.... GTAO and RDO won't really compete with each other. Sure, many people will move on to Red Dead and stop playing GTAO (maybe really forever or at least for a while) but the GTAO fanbase is so huge that half of the players could suddenly stop playing it and Take Two would still make tons of money with little development effort. Can't say I understand why this 4 year-old game is still printing money but Rockstar would be stupid If they'd suddenly stop releasing updates.