Rockstar games makes more now than ever before so of course, there will be GTA VI. The graphics are getting better all the time and game experience becoming more realistic.
I think there is going to be new games. The franchise is going to be good enough. I think they have lot to offer. They can also make revenue from the cars shown in the game. So there is something that you may want to find out in years to come.
Although this thread is four years ago, I just want to let you know that there will be a GTA VI. Grand Theft Auto 6 has not been announced yet, but we are very much hoping for some concrete details this 2018.