That's not emulation that's decompilation / recompilation. Which is a very challenging task. TransMeta worked on this under the name of "code morphing". It's a promising technique but I don't think it has the maturity to be able to run cross-architecture code at close to hardware speeds (say, 50% or higher) without introducing new defects, especially x86 on ARM.
What you are discussing about, is a runtime issue.
I was discussing about installation time, which only prevents situations were you make use of code rewriting tricks, which are anyway frowned upon in the day and age of read-only code sections and no-execute data sections.
