> attempting to browse the modern web with a
  > 90s browser is futile

I think the inverse is what we're talking about. Will we be able to browse NaCl sites in future browsers (possibly after an architecture change away from x86)? As it stands, we would probably be able to browse a 90's site on modern browsers.

It sounds like if NaCl compiles things down to ASM x86 (or really AMD64) (?). If that's true, then it's possible for things to not be forward compatible. I'll admit that it doesn't seem likely, especially after what happened with Itanium and AMD64/x86-64, but we could see things move towards ARM.