https://news.ycombinator.com/item?id=33974265 (678 points/753 comments)
This will weaken the defences of the walled garden, unless Apple mark-sweep the 3rd party stores. If you value a walled garden, and some people do, then the walls got lower. This isn't necessarily good.
I expect to see ringfences go up: Safari and Mail and like apps may well stop doing open file on content from side loaded apps, if they can't prove they are safe. (proof.. whatever that means)
But Apple experience has been far more constrained and it's single source with almost no prior of sideloading. They're a different set. I am less sure how any sideload in Android or install alternate marketplaces is informing, maybe it's the best information we have.
The only apps that are going to be available on third party app stores but not on the official one are going to be apps that would have never been available on iPhone at all in the first place if not for this change.
We already have a real world example of this exact situation with Android, and there's no real store fragmentation there.