There are bad actors, like Facebook, who will prompt users to sideload so they can do bad things.

If the bad actors were all shady fly-by-night looking companies it’d be one thing. But once the gates are opened, if it’s easy enough, you’ll see people being pressured to open the gates by all sorts of companies that they will have no choice but to comply with. Imagine, for example, if your insurance company, in order to pay your bill, starts making you install the app through some alternate pathway that brings a side-dish of keylogging to adjust your rates based on your browser behavior.