GTA V sales are as strong as they are due to PS4 sales. They both sell at pretty much the exact same rate, meaning that when people pick up a PS4 they usually also pick up GTA V. So due to GTA V's advantage with the PS3/360/XB1, I think it wins by default.
There is another scenario where RDR2 or GTA VI replaces GTA Vs sales, but I think that is compensated by the inevitable GTA V remaster in 10-15 years.







