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’ve enjoyed the 2 cruises I’ve taken but I’ve also enjoyed DFW’s famous satirical essay about cruises. I like to enjoy both middle-class and elite snob pleasures. https://harpers.org/wp-content/uploads/2008/09/HarpersMagazine-1996-01-0007859.pdf