DRMed mobi files will just become obsolete because of its exclusivity. (Except the Amazon format, of course)
ePub will just replace it. ebookstores will have to support it, because more and more people will have ePub ereaders or will read in ther computers using Adobe's software, which is more known and popular.
Current users will just have to endure this transition phase: by removing drm from their ebooks, while avoiding drm mobi ebooks.