Let's compare the x86 ISA to, say, a Ford Model T, and a reasonably modern RISC ISA (SPARC, PowerPC, MIPS, etc.) to, say, a Ferrari. The latter is newer, prettier, and inherently faster. However, this Model T is so successful in terms of sales that Ford has been able to hire a bunch of engineers to figure out how to add a bunch of structural reinforcement and replace the original engine with a couple of turbojets. Now the Model T looks even cruftier than before (it's got jet engines bolted on!!) but it's now at least as fast as the Ferrari.
The problem with that analogy is, the mainstream Fords never got faster than a Ferrari (although in both cases, you'd be a moron to rely on the Ferrari to get to work every day). Intel and AMD have done a good job at leaving SPARC and PowerPC in the dust.
