There's no reason why NaCl should be inherently limited to x86. x86 is currently the biggest platform, but there's no reason why NaCL couldn't switch to a "Universal" payload like OS X has, combining the 2 largest ISA. (x86 and arm?)
EDIT: I've since read that NaCl is working on LLVM, which would be even better.
2. A 'universal' payload with ARM and x86, would just work on them. What if in 5 years we have new architectures? Only supporting ARM and x86 would hold back innovation there.
2) You're posing a what-if for 5 years down the road? Your horizon is really that far? Sounds like grasping for straws to me.
GP was right, including just x86 and ARM is not good enough. Heck it even ignores x86_64 right now, and won't support new SIMD extensions that come out from time to time. Not to mention be a barrier to anyone introducing a new arch, as GP said.
Thinking ahead here is vital.
It's like trying to run a C64 binary today. The only practical way is emulation, which is slow - but thankfully fast enough in this case. In general though, it means new architectures will run more slowly than existing ones. That's not a good thing.