As far as I can tell the Icon of the Seas is basically fine; even on the environmental front (important!) I see no reason to believe it's worse per passenger than other ships.

A lot of opposition to it feels like elite snobbery against middle / upper-middle-class people.

Any good anti-Icon arguments I'm missing?

I see a lot of people mistakenly comparing this to seasteading, when these types of ships are in no way self-sustaining and require all their supplies from the land. Let's see a cruise ship with less recreation and a vertical garden, farm,  industrial manufacturing plant in their place.

The goal can be how long the ship can stay independent at sea.