Before Sony decided to support EPUB via DE, I'd have said there was at least a chance (though a small one) that Sony might decide to switch completely to Mobipocket rather than their own proprietary format. Mobi might have gone along with that, if allowed to do so by Amazon (rather doubtful, but possible). Now... I doubt Mobi would do it.
And I suspect that the ebook reader vendors who've gone with Mobipocket have had to sign contracts saying they won't license another competing DRM scheme for their devices.
Which strikes me as a practice that ought to be illegal, but MS has been getting away with it for years.