Take the extreme case, lets say litecoin is around in 15 or 20 years, are you really convinced that GPU's won't get cache or threading in that time?
I honestly don't know.
But that depends on what tasks a GPU is supposed to do in the future. If the basic rendering techniques don't change I don't see a reason why they didn't just pack more and more ALUs on the chip and leave the architecture as it is.
If not... how we are supposed to tell what's coming?
There might be a future in voxel graphics and raytracing both which would benefit from a different architecture and it would make sense to optimize the hardware for that if they ever become popular.
The thing we ought to be looking for are sandwiched/3d chips where a memory die could lie on top of a alu die, eliminating the huge effort to produce cache memory, if that ever takes off even small microcontroller like devices with multiple alus could outperform todays cpus but I have no Idea when the gonna be.