I've not sure where you've seen them say it's not eventually coming, they've left that possibility wide open. Direct quote from the president of Rockstar North from a month ago:
Interview: Rockstar North president Leslie Benzies on GTA Online and GTA V - ComputerAndVideoGames.com
From July:
GTA 5 On PS4? Not So Fast, Rockstar Says
Also July:
GTA V Requires 8GB Install, No Difference Between PS3 And Xbox 360 Versions
I think it's only reasonable to assume that a higher frame rate & resolution version will eventually come to next gen consoles. Will that be any time soon? Probably not, they want to maximize sales of the current versions, so they can eventually get people to double dip on the next gen edition. Certainly no reason to wait, but you can pretty much guarantee you'll eventually be able to get a better version on next gen systems.
Q: So do you think the next generation will help and allow you to expand your plans more easily?
A: We'll get the current gen version out first and see what happens in the future.
Interview: Rockstar North president Leslie Benzies on GTA Online and GTA V - ComputerAndVideoGames.com
From July:
"The only versions of the game that we have announced are for the Xbox 360 and PlayStation 3 which are set for a September 17th worldwide release. We don’t have anything to share about the possibility of a next-gen or a PC platform release at this time and we are completely focused on delivering the best possible experience for the consoles people have right now."
GTA 5 On PS4? Not So Fast, Rockstar Says
Also July:
The game has only been announced for PS3 and Xbox 360 so far, in spite of evidence of a PC version. The studio declined to comment on PC or next-gen console rumors at this time.
"We are completely focused on delivering the best possible experience for the consoles people have right now."
GTA V Requires 8GB Install, No Difference Between PS3 And Xbox 360 Versions
I think it's only reasonable to assume that a higher frame rate & resolution version will eventually come to next gen consoles. Will that be any time soon? Probably not, they want to maximize sales of the current versions, so they can eventually get people to double dip on the next gen edition. Certainly no reason to wait, but you can pretty much guarantee you'll eventually be able to get a better version on next gen systems.