I feel it is the way to go, to an extent. The EU forcing Apple to adopt everything that android has would make everything too saturated. iPhone clones would rise to an all time high, and there wouldn’t be many things to differentiate between iOS and android. Though I am quite interested in what others have to say.
I’m generally in favor of allowing sideloading. It shouldn’t be enabled out of the box, but it should be an option you can enable from somewhere buried in settings.
It wouldn’t create iPhone clones because sideloading doesn’t allow you to install iOS onto a non-Apple device. What’s more, sideloading is not the primary differentiator between iPhone and Android. iPhones still run circles around their Android counterparts in terms of performance, iOS still has a superior app ecosystem (fewer apps, sure, but there isn’t a sea of crapps), and it’s like Android devices seem to copy Apple’s style.