Is it possible to allow sideloading and keep users safe?

On systems before apple's locked-down iphone, it was just called "installing".

The PC revolution started with people just inserting their software into the comptuer and running it. You didn't have to ask the computer manufacturer or the OS vendor permission to do it.

And note that apple doesn't allow you to protect yourself. You cannot install a firewall and block arbitrary software on your phone. For example, you can not block apple telemetry.

Apple also has enforced a similar policy to what Google is doing, but much stricter, and has done for ~13 years or so (devs must be identified, the OS rejects unsigned code in all territories by default, Apple pre-approves all binaries even outside the app store).

Linux distros have policies far more extreme than anything Google, Apple or Microsoft have ever done. They explicitly don't support installing any software not provided by their "app stores". Getting into those requires giving up your source code to them, and they reserve the right to modify it as they see fit without informing anyone, reject it for any reason or no reason at all (including reasons like "we don't have time"), and they tie getting new releases of your app to the user upgrading to new releases of the OS. If you do try and install stuff from outside of your distribution, not only are there security warnings to click through but an expected outcome is that the OS breaks and the vendor washes their hands of you.

Despite those policies, or perhaps because of them, botnets of Linux servers are common.

Of all consumer-facing platforms only Windows and Android allow installation of unsigned third party code out of the box via some obvious graphical path. And on Windows that right is somewhat theoretical. You can do it but the built in browser will try very hard to stop you, and the OS itself will happily break unsigned code by blocking file open syscalls heuristically. So in practice most apps don't go the unsigned route. On Android OTOH, unsigned (non ID verified) code is sandboxed and works just like regular apps after installation, the OS won't heuristically interfere with the app.

Most Linux distributions don't prevent you from installing third party software at all. You download something, you set the execute bit, it runs.

Users are wary of doing that with software from untrusted sources because, obviously, you're then placing your trust in whoever provided the software instead of the distribution's packaging team. But the OS won't stop you if that's what you want to do, and sometimes you do trust the source of the software.

> Despite those policies, or perhaps because of them, botnets of Linux servers are common.

Botnets of Linux servers are common because some people operate them without security installing updates (common with WordPress), and then attackers exploit known vulnerabilities in the unpatched software.

But "locked" phone platforms regularly discontinue security updates for devices that are still in widespread use. Locking the device doesn't solve that problem at all, and in fact makes it worse because then if the OEM doesn't patch it nobody else can do it either.

The OS doesn't stop you installing third party software - signed or not - on macOS, Windows or Android, so "allow" is nothing interesting. That also won't be changing with Android, given that you can buy a phone with an unlockable bootloader and reflash to some other spin of Android that implements whatever security policies you want. You can put these devices into a mode that allows anything.

The question is whether that's something the vendors make easy, if they support it in the sense that you can do it and they will still deal with you if there's a problem. That's what support means. It's not a synonym for technically possible.

Windows, macOS and Android don't consider installing third party software to put the system in an unsupported state. Linux vendors do.