NVIDIA long term support is fine, IMO. AMD cards have historically gotten better with driver updates as AMD learned to wring more out of them and I wouldn't be surprised if the fact that the consoles basically have a ~7870 in them meant that some games wound up more optimized for GCN than Kepler.
Maxwell seems to be a good architecture and Pascal appears to build off of that architecture, so it'll be supported for quite a while.
Maxwell is actually a bad architecture for modern compute-heavy games. The more compute based effects you pile on, the worse it runs, as you may know, the slow context switch hurts it.
Pascal is already better in this regards with a fast context switch.
Example, Far Cry Primal (neutral non GameWorks) went away with using tessellation-based God Rays & Fur that FC4 (GameWorks) did, they used compute based effects and that's why GCN runs it so well compared to Maxwell. Same engine as FC4! Just a switch to more compute & less tessellation.
If you want to talk about the next few years, will more compute effects be used or less? See, I would lean towards more, since that's the trend.
If it's a compute-heavy future in games, then for certain Maxwell will tank relative to Pascal & GCN.
Btw, I am not talking about Async Compute, which is a different issue.