I don't expect the EU to fine Apple enough that it might actually hurt them, but if we suppose the regulators wanted to, is there anything to stop them?
The highest GDPR fine was 1.2 billion. As far as I know nothing is stopping the EU from imposing higher and higher fines with continued breach of guidelines there, and I would expect these fair market regulations to work similarly.
Also for reference, that fine was against meta, who had 34 billion in revenue in 2023. So that fine cost them around 3% of their global revenue, which I'm sure is tolerable, but definitely approaching the point of hurting.
I am by no means an expert but this seems like a ludicrous response from Apple.
They can’t take this fight as, like you say, pissing off the shareholders will force them to change direction; if the EU do start talking about repercussions.
10% of revenue is going to hurt. That said I din't think Apple will budge until that fine is hammered down on them. They don't seem to have enough foresight to walk the tightrope succesfully.