I've thought about this too, but I don't think it's a big issue. Pools can always split and change size as new people enter and leave. We're already seeing this arms race as the BTC difficulty retargets have been around ~+20% the past few times. Eventually though, it'll reach an equilibruim where there's simply no profit in manufacturing new ASICs and new players can just amass existing hardware to compete. It's possible we'll watch another monopoly unfold once that happens, but it's much too early to tell.