It will be inconvenient and even impossible to migrate some things (for example contacts, iMessage history, app data). Espcially consider how difficult it would be for a novice user to migrate. And what are those other choices? Presumably you are not considering Android devices.

It's not Apple's fault if Android devices aren't suitable alternatives.

Of course not. But the limited selection of alternatives certainly adds to the pain of switching platforms. Therefore it's not so simple to "just buy another phone".

I wholeheartedly agree with that. I just find it difficult to blame Apple when the accusation is that the competition sucks.

That's not the accusation. The accusation is that disallowing sideloading and forcing all customers to observe a single philosophy is especially big of a risk to the user considering that there's little competition. The fix isn't for Apple to create better competition, it's to allow sideloading. I am not saying it's their "fault" for having a lack of competition, that is just a reality of the market they are in.