They do, but they also get a lot of new cruisers also. The newer ships on the traditional 3/4s, helped pay off the Dream class early. I want to say the initial plan was for 5 years and it ended up about 3 years.
Disney was limited by the number of ships, but also there own lack of imagination. Carnival and Princess both run 7 day plus southerns out of Miami and FtL. Doing Aruba, Bonaire etc. One of the DCL Capts was asked why Disney didnt do something similar. His response was we cant do that with out PR. Really? Explain Princess 6 and 8 day southern trips, that dont stop, end or begin there. Disney had the perfect opportunity several times when there was 3 ships in PC for several weeks, several times to at least try them and see what happens. They did additional 3s and 4s instead. 5 day trips could have been thrown in the mix also.
Yeah that's what I mean why I say I'm their bread and butter. They seem to try and get the ones who are still happy with being on those "common" itineraries. And while they will never stop doing that, and shouldn't, there's now a lot more clientele who are itching for something more. Eventually, when you have a large chunk of your base who are passionate Disney cruisers and have been on all four ships, you have to expand a little bit.
I will say that I do hope they have more five nights. 3 and 4s are too short and on the last 7 night I went on, it felt too long by night #6. 5 nights would be perfect, especially since you could do a double dip at CC (or the new island) or a 5 night West Caribbean. Carnival and RC, probably the two biggest competitors, have a good amount of 5 nights. I know the Dream is doing some of those in 2022 so hopefully those will happen more often. Like I said above, there's plenty of people who would want more, but I personally didn't like how they had 3, 4, or 7 for a while.