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.
X86 is amazing in spite of legacy and poor architectural choices.
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.