Apple started this incarnation of selling songs, and they will surely see it through to the end, especially once they bring it to Windows. At that point, even if other companies jump on the bandwagon, they will be a competitor for your dollar, and if their service is anything above decent they should stay above the crap. Inevitably, as some innovate, Apple and others will imitate, so we should see nothing but good come out of this, especially if companies battle to lower song prices.
If for some reason Apple sales started dropping drastically, I have no doubt they would eventually have to make their processors compatible with Windows code in order to sell machines. After all, the bulk of their sales is hardware, so that is upon what they subsist.