The x86 thing is just grousing by people who think aesthetics in the assembler are the definition of a clean architecture. Instruction decode for a modern x86 CPU is indeed a difficult problem when compared with cleaner architectures (though ARMv7 is hardly "clean" -- how many instruction sets does it support now? Five, I think?). Instruction decode, however, is one of the easiest problems to tackle on a CPU. It just happens to be the part that software people understand, so it's what they like to whine about.
You're absolutely right: if it could be done much better, there'd be an example in the market to prove it. Yet Intel is walking all over the market, and has been for the last 8 years or so.
Statements like this -- saying that x86 is either good enough or as good as anything could possibly be anyway -- sound to me like a lack of imagination, lack of perspective, or not wanting to stir up any cognitive dissonance given that market forces have caused x86 to dominate.
Would you also say that there probably couldn't exist a significantly better OS than Mac, Windows, or Linux, or else we'd know about it? I admit "better" can be hard to define; what would make a 10x better car, or IP protocol, for example? It strains the imagination, because what makes these things "good" is complicated. But ask anyone who was around during the early proliferation of computer architectures and operating systems, and they will tell you that what ended up winning out is markedly inferior to other things that could have been. Paths get chosen, decisions get made, billion-dollar fabs get built. The market doesn't pick the best technology.
It's like web standards -- accept them, but only defend them to a point. We may be stuck with CSS, but that doesn't mean it's any good or that it succeeded on its merits.