If some code runs orders of magnitude slower it's the same as being incompatible. Yes you might execute an ASM.JS game on older browsers but at 1FPS it's not enjoyable.
Yes, but it seems the blame game plays out differently. If a website won't run because it's demanding a plugin of some kind or a different web browser then the user tends to blame the website; if it runs, painfully slowly, on their browser but quickly on a different model of browser then (s)he tends to blame his/her browser. Really, the way forward is to have things set up so that sites can compile to arch-specific NaCl and PNaCl and "asm.js", and then serve the browser the best one that it will accept.
