Carnival doing Europe is def not a common thing, but a little more frequent than in the past. Whether that. mAkes it an oddity I have no idea. I think the reason that cruisers do sail them in different and unique cruises is their desire for the familiarity of the cruise experience (our personal reason). That said, can I understand where some might find thees things a plus? Sure. Which is better is up to each of us to decide. A Princess, HAl or Celebrity cruise will not be the same cost as a Carnival cruise.
The Costa comment is an interesting one, from two perspectives. Do you feel the same with MSC? Also, Carnival taking two Costa ships and rebranding them as the Carnival Costa experience will be interesting to see how American cruisers adapt to that. Interesting discussion