It's a race to see which one gets the killer app, the one everyone has to have, and then the other platforms will drop off, and there will be a consensus platform, the one "everyone" develops for. I hope, for the sake of all of us, that it's a platform without a platform vendor -- that no one gets to decide who has access to the market and who doesn't. Of course if that ever comes about, and the vendor throws around their weight too much, that's when the DoJ steps in, assuming the superrich haven't completely bought the government by then.
Anyway, it seems to usually happen that way. There were lots of personal computers until Visicalc came along, then the consensus was the Apple II. Until the IBM PC came along and everyone wanted to run Lotus. Then Windows and Excel, and the web and Yahoo, Amazon, Google, etc.