NaCl is one of those technologies that I secretly hope fails. Pull the plug on x86 already, FFS.
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.
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 is good, but precluding a technology because a particular implementation would have to be changed in 5 years is just flat-out silly for tech. You couldn't ever buy a car with criteria like that. Grasping for straws.
The point is that if you have only x86 and ARM binaries, you can't run them directly on a new architecture. It isn't a matter of changing an implementation.
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.