The ruling could cost them billions of dollars in revenue. It's absolutely worth appealing. Even if they lose in the long run, they'll make a few billion more in the mean time.
how many billions? It seems their max app gaming revenue lost would be 64 * 70% * 30% = 13.4B for 2021 , but it's probably a lot smaller due to special deals, and also this would assume they would lose ALL the revenue. In reality they'd lose about,maybe ~$3B ?
The alternative would alienate some of their best developers -- who knows, maybe they'd leave?