Losing 7% would be "A big deal", although they'd also lose a lot of the operational costs so the overall loss may be less than that. There are several costs of doing business in the EU.
I can also see them selling more to the UK and a black-market opening up in the EU for UK phones. Again, not a huge thing but a little +ve offset.
Overall, I could see it happening, maybe, emphasis on the maybe part. I think the EU thinks it's inconceivable for Apple to exit their market. I also think Apple are very hard-headed. Make the cost/benefit balance skew too heavily in the -ve direction, and the potential of future profits stop outweighing the costs.
Personally I really hope they just do as they're told by the EU because I think it'd hit the stock, and I could do with the stock staying reasonably high in the short term, but they'd recover. Hell, they could run on cash/savings with no sales for several years at the current rate...
[1]: https://daringfireball.net/2024/03/more_on_the_eus_market_mi...
Eh.