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?

there's the sheer (rising) cost to these vs the lower demand to pay for them, the logistical dead end of optimizing for 5k ppl over a week, and it's a vehicle that doesn't transport passengers over long distances.