People don't realise how many things have CPUs these days. PPC (not POWER) and MIPS are also still within the same magnitude as x86 chips (hundreds of millions/year) - they both used to be selling more than x86 because they were more widespread in embedded niches (networking, automotive, set-top boxes), but I don't know whether or not that's still true or not.

It's not economies of scale that gives Intel its sustained advantage as much as it is huge margins that allow them to continue re-investing in their fabrication process advantage, which again protects their margins by keeping them on top in the only segments of the CPU market that are both high price, high margin (at east for the top SKUs) and high unit.