More importantly, content using NaCl would not run on any new hardware platforms that might appear.
So if the web were to use NaCl to an appreciable extent, new platforms would be unable to get any traction in any context that relied on the web. We'd be stuck with ARM or X86 forever for any mobile devices we might have.
Maybe you think that's ok; I think that's a terrible idea; an attitude like that toward the web in the late 1990s would have meant that ARM phones that appeared in the late 2000s would not have been able to browse the web!
(Also, note that I said "hardware _platforms_", not what you asked above.)