If Apple halts their iPhone sales in the EU, they endanger their platform globally. Because suddenly a pretty large and important market would go 100% to Android. Also, Macs would become extinct pretty quickly in the EU.
The fine initially would be up to 10% of the revenue. I don't see how that is outrageous if the violation of the law is there. The actual fine could be lower, but on the other side, the fine needs to be high enough that it can force the fined company to stop breaking the law.