I get that those traveling from afar are not going to be the "bread and butter" but I think in addition to losing American customers with these itineraries, they also completely lose Europeans, and even closer to Singapore, I think they largely lose Chinese and Japanese cruisers.
I've been on the MSC Bellissima out of Taipei, and our two port stops in the Okinawa Prefecture seemed very popular, with a high proportion of passengers seemingly taking MSC excursions. Even with a cruise that largely catered to passengers from nearby countries, for whom language wasn't a high barrier, for whom there wasn't a high cultural barrier... I still saw tons of passengers with booked excursions, telling me that many travelers there still liked the comfort and ease that a cruise excursion provided.
You probably wouldn't be able to tell those passengers, hey, come visit Singapore, take a cruise, and then if you actually want to get out and about in the region, take a short regional flight, figure out transportation in a new strange town, change your currency, find a hotel, etc. Basically, the people that are already opting in to cruises are doing so because they feel this is one of the best ways to see the world, via a floating hotel that takes you to your next destination. You can't convince these people to prioritize a cruise to nowhere, because then they'll never see more than the port city. China, Korea, and Japan have rapidly aging populations. These cruise passengers still want to see the world, but they prefer a cruise ship for the ease of travel. I think Disney is going to take a big ol' swing and a miss on this demographic.
So Disney is going to lose some percent of Americans, Europeans, as well as mobility-limited Chinese, Korean, and Japanese customers. That starts to become a lot of segments that they're not going to appeal to IMO.
I wouldn't be surprised if they change their itinerary strategy pretty.