Those are excellent observations - I haven't been following the ships that closely. At least their histories. That being said, I don't see Disney committing too much more to the Caribbeans for a number of reasons (beyond the obvious super saturation).
1.
DCL's marketing startegy is pretty straightforward. Get families into cruising with a three- or four-nighter aboard Dream out of PC. (Get them while they are at the parks.) Move them up to a 7-nighter in Fantasy or another longer itinerary in the Caribbeans - and then get them to splurge on the itineraries with fat profits (Alaska + Europe). Most North American families won't start their DCL cruising in Alaska or Europe (unless accompanying someone else), but the lifetime value of a DCL cruiser climbs astronomically once you can get them out of the Caribbeans.
2. The funnel. If you have too many low-margin itineraries, you will have people hooked on to cruising, but they will then flee to other cruise lines for better itineraries. You want them to stay with you when the real spending comes. (You want to grow - no pun - with your cruising families.) Unlike other cruise lines, DCL doesn't have millennial vs experience brands - so the segmentation must come from the cruising experience itself.
3. Let's do the numbers. Let's calculate the fares of a verandah room with four occupants for the combined months of July and August 2019. I would use the 2018 numbers, but several Wonder verandahs are sold out for those dates.
Disney Fantasy: $4,340 + $9,841 + $8,662 + $6,760 + $9,757 + $9,964 + $8,020 + $5,369 + $13,523 = $76,236
Disney Wonder: $10,496 + $14,732 + $14,539 + $14,217 + $13,755 + $12,817 + $11,826 + $11,099 + $18,184 = $121,665
Now, let's look at the availability in July and August 2018. Categories include inside, oceanview, verandah, and concierge.
Disney Fantasy: Out of 4 categories x 9 sailings, 5 are sold out.
Disney Wonder: Out of 4 categories x 9 sailings, 16 are sold out.
So - DCL charges ~60% more on Wonder, yet four times as many Wonder sailings are sold out. That's a staggering demand/supply imbalance. (Caveat: Fantasy has ~42% more rooms - but also a disproportionately higher build cost.)
4. There is nothing to 'try out' any more in Europe and Alaska. These are established cruise markets. Princess, for example, will have nearly a dozen ships deployed in Alaska next year. The modifications required are trivial compared to the profits. Yes, DCL is not a trend maker, but going to Alaska and Europe in 2022 is a much belated trend
following.
5. Disney has shown tremendous interest in Asia. It now has three - not one or two but three - theme parks in Asia Pacific. And they are ringing in big numbers. Disney just need to push through another 4 or 5 years of movies - just in time for the last of the new ships to roll out.
6. Completely agreed about the LNG ports - but most of the fully developed LNG ports are currently in Europe. Spain (such as in Barcelona) is investing in LNG fueling capacity, and Germany is way ahead in that respect.