Google puts up tons of scary warnings for sideloading…and IIRC, that’s part of Epic’s case against Google (the warnings discourage people from installing alternative stores or from alternative means), in addition to Google’s position on Play. (I do not believe that Epic has a meaningful case on any front, but that’s ultimately for the courts to decide. The courts often decide wrongly, as was recently done for Oracle v Google re: the copyright of status of APIs.)
You say that Apple can make you pay extra. OK. Here’s a $99/year developer contract with which you can develop and install software that you want as you want (I believe that these builds are good for ~90 days, so you recompile/reinstall every 90 days; unlike the 7 previously mentioned). But people don’t _like_ that and have said that’s unfair.
I am completely saying that there’s no _meaningful_ way we can put up a fence to keep people who _shouldn’t_ be running random apps from doing so. We can’t keep users from clicking on _links_ that they shouldn’t be clicking on. Just this morning, I had a neighbour ask me about one of those full-screen “WARNING FROM MICROSOFT YOUR COMPUTER IS INFECTED” pop-ups; even though she was smart enough not to click on anything, she _still_ copied down the phone number to maybe call the scammer. My father, a couple of weeks ago, didn’t know about Ctrl-W / Command-W on a similar full-screen hijack that had affected both his Chromebook (locked down) and my mom’s MacBook (mostly not locked down).
I remember a few years ago there were a number of minor malware issues that were caused by people following instructions randomly on the internet to open the javascript console and “paste this in to see something neat”. So no, I don’t believe we can put a fence that protects the ignorant / unready / unwise but enables the people who think that they know better (and actually sometimes might).
If you can crack that, then there’s going to be a lot of people who will be at your door to reward you…and then many more looking for the backdoors you left so that they can continue to infiltrate systems for their own rewards (whether state actors or criminal actors).