I don't doubt they could get 50 years... but I doubt they do much beyond 30 years.
It's a numbers game and smaller ships are more costly to operate on a per passenger basis. Magic and Wonder both have a limited number of those all important balcony rooms, so it's hard to make up the difference by charging "premiums" - they have been able to, by offering these ships on special itineraries. But if a bigger ship could do those itineraries more profitably?
Then two they both burn the less ecologically friendly fuel oil - that likly will put a limit on the life span of these ships. Both from an operations standpoint as these fuels will likly become more costly, but to as guest start to take more note of the smoke coming from those stacks... especially when in port.