If they leave the EU market, Android immediately becomes a much bigger focus for every app developer which can have an effect on the quality of apps available on the app store around the world.
https://www.statista.com/statistics/639928/market-share-mobi...
However, more importantly, iPhone users have more disposable income [1][2]. So, if Apple would pull from the EU, all those iPhone users with a lot of disposable income would be in the market for Android phones and apps, creating a lot of opportunities in the Android market.
[1] There are various studies, e.g.: https://www.prnewswire.com/news-releases/iphone-users-spend-...
[2] If you look at the most popular Android phones, they are often dominated by cheaper models, e.g. Samsung's A lineup. See e.g. current Android usage in France: https://www.appbrain.com/stats/top-android-phones-tablets-by...