I get some of your differences, but some are not unique to Disney. We have cruises on 5 different lines, and while we enjoy DCL they aren’t some unicorn with so many specialty features no one else has.
1.
Castaway Cay—we love it but most lines have a private island. So that’s not unique. Castaway Cay is different in not as developed feeling, but it’s till ocean, beach, and food.
2. The NCL ship we were on had a full walkable promenade with 4 outdoor bars and outdoor restaurant seatings for 4 restaurants (this isn’t counting the pool bars and food areas). So a promenade is not unique to Disney.
3. no casino—agree.
4. Adult only areas onboard and on the island. Literally every cruise line has these. Not unique to Disney.
5. Disney entertainment—agree and that’s the ace in the hole for them.
6. Fireworks—agree
7. Decorations—haven’t been on other lines over the holidays. Saw plenty of decorated doors on other cruise lines. On NCL people hide rubber ducks around the ship for people to find. So not unique to DCL
8. Spacious staterooms—again not unique to Disney. Every line has them.
9. Aqua Duck—while the other lines don’t have the Aqua duck they have plenty of other water slides that DCL doesn’t have. We had 5 on our last cruise. Royal has tons of different ones. Most of the newer ships on other lines have more than 2 slides which is what the dream/ fantasy has.
We enjoy DCL but I also have been on other lines enough for us to know that DCL isnt head and shoulders above everyone except for the kids club for our family. So we are open to other lines depending on what we feel like. Especially for the price. But everyone is different and enjoys different stuff.