The way this is designed appears to be entirely intentional and sensible. Yes, you can still install malware using ADB. It's just harder. That seems really sensible. It's a speed bump, but not a prohibition.
I also haven't seen any specific examples of software that's frequently sideloaded that would be unjustly discriminated against.
I also haven't seen any specific examples of software that's frequently sideloaded that would be unjustly discriminated against.