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With the third-party appstore also being available in the EU, someone will surely roll a non-webkit browser, though I have no idea how hard that is going to be as I don't develop for iOS.

You are not wrong and that's probably what Apple is counting on. Especially since if you rely on services from outside the EU then those people can't really test their website on the non-webkit browser engines. Of course if more countries force Apple to allow 3rd party browser engines the scales could flip.

A lot depends on expectations.

If Google expects these rules to be imitated globally over the next 5-10 years it might make sense to invest in a non-WebKit based implementation.