If you sell your new platform as "open" to gain market share and then break that promise of openness after you successfully drive competing platforms out of the market, that sounds a lot like fraudulent marketing to me.
But, ideally, it's not that big a deal, because the community (or even newer non-calcified businesses) can fork the last open source branch and continue development.
Unfortunately, with something as complex as an OS, that's incredibly difficult. It does seem regrettably unlikely that, for the foreseeable future, there will be no practically usable open-source phone OSes.