This seems necessary and good. There are occasional bad actor apps that slip through Google's app store review process and there are good apps that get sideloaded, but sideloading entails literally zero review. It's entirely up to the user to determine whether they're trustworthy or not, which is a paradigm shift for most who don't think phones could harbor anything worse than just obtrusive ads in apps.

There are plenty of good apps I've sideloaded, but thinking of the majority of the population being relatively tech-illiterate, there's a big difference between a bad actor having to convince someone to change their settings to enable sideloading (despite a big, scary warning) and then convincing them to install an app versus just convincing someone to run/install a random APK. Case in point: a huge percent of the population unknowingly opens and/or installs random bad binaries on Windows every day.

Sideloading (and all that entails) should absolutely be allowed, but the "scary warnings" do a great job preventing a huge portion of people from unknowingly opening a huge threat avenue to some of their most valuable devices/information.