IMO,
DCL is a niche cruise line that will maximize its revenues by maintaining a small fleet that provides a Disney-experience. It's clear on this forum that people place value in flying into Orlando, riding Disney busses with Disney entertainment (obviously not the case status quo) to the port, traveling through a Disneyfied cruise terminal and visiting Disney-created ports.
European cruises generally sell at a strong premium (compared to those in the USA). They also cost the cruise lines significantly more to operate, and that's especially true of Disney which doesn't have the privilege of bulk procurement. Yet Disney's European cruises sell for roughly the same as their Florida counterparts (even on the Magic and Dream next summer). Not a market I'd look for additional capacity anytime soon.