No other passengers are dictating what other passengers wear. The dress codes for the various venues are dictated by the cruise line. They know what kind of ambiance they want their venues to be and set a code accordingly. When that code is set that is what patrons should expect the standards to be.
Pretty simple, or at least it should be..
If you haven't done so already wander over to the Caribbean ports of call forum. There are sub-forums for all those places with great excursion/transportation info. Here's the link for you:
https://boards.cruisecritic.com/forum/456-caribbean/
It's not just MSC, all the cruise lines are scrambling right now to find the best possible solution to this dilemma. I understand your angst but this calls for patience at this time.