Signing bins as part of your build chain seems easy to me.

Don't self-signed apps expire after a week? You'd need to distribute a new build every week for people to continue to use your app. That's not a major hassle if you're just using your own code, but if you want to distribute something to other people it quickly becomes very problematic.

You can "sign" your binary without any identity associated with it and macOS will let it run. There is no time limit on those.

I agree - for commercial development its a trivial step. But it's yet another barrier to entry for students and people that just want to hack on their own computer at will.

It's also a potentially dangerous first step towards forcing new versions of MacOS to only run code from verified developers - those that pay Apple. Again, trivial for commericial development, but a major issue for non-commercial use.