Market share argument:
Android has 71% market share in the EU, as of February 2019 - compared to iOS's 21%. [1]
Ease of customer switching argument:
If Apple pressed too much in a way that harmed consumers, there are many other cheaper alternative Android phones vs. iOS to which they can easily port their number over (say what you want about ISPs and Telecoms, I find this to be pretty easy in the US - unless you signed an unreasonably stifling contract. Admittedly I'm unsure how difficult this would be in most of the EU.)
Supply-Side Price elasticity argument:
In theory, if the 30% cut was too high, developers wouldn't want to develop on the iOS platform, and would instead focus resources on developing on Android; the specific cut would be at an equilibrium point that enough developers would find profitable, and would still provide choice in the app marketplace for customers.