I think they could do something like what Tik Tok did, by letting users know why they can no longer provide the service.
I would personally give Apple money to see them actually stand-up to this. What's probably more concerning is the number of companies not complaining about this at all.
This headline comes to mind: https://en.wikipedia.org/wiki/Enemies_of_the_People_(headlin...
I don't see UK judges getting motivated to rule in favour of a foreign company because they took their ball and went home, not even in cases where the ball happens to be very popular.
Governments are extremely powerful. They may issue a gag order (https://en.wikipedia.org/wiki/Gag_order) that makes it illegal for Apple to do that.
If push comes to shove and apple actually called their bluff and withdrew completely from the UK market, I'd bet that that government would become so unpopular that they would not be elected again for quite some time.
Seems absurdist. They have to implement the backdoor, appeal, and only if the appeal is successful can they disable it.
Here in Spain no person has even tried to SMS me (which is the fallback for iMessage which I don't have) for 6 years or so :). I also don't have RCS enabled. It's all WhatsApp and Telegram.
Unlikely. That's illegal.