Of course, there are issues to deal with here, both in terms of the extra burden snapshot isolation places on tempdb, and the fact that swapping to this mode may cause some issues with existing applications that weren't designed with it in mind. However, as these issues are planned for and addressed, I envisage snapshot isolation becoming the norm.
Please! When is this going to happen? In fact, when is the shared tempdb going to be removed entirely? If anything stops SQL Server from scaling with multiple databases on the one instance, I think it's the tempdb.