The false assumption in your argument IMHO is the assumption that none of the software on your device will ever betray you or contain an exploitable security hole. In actuality, it is useful from time to time to be able to run software you cannot completely trust such that the software cannot access all the data on the device (because the untrusted software cannot access your enclave).
That's why you run that software as its own untrusted user and perhaps run it with some kind of sandbox. It's not a reason for you the owner to not have root access at all.
Running each app as its own untrusted user is one of the measures taken by Android, but the designers of Android do not consider that enough, so they also sandbox the app with selinux, but no one has implemented sandboxing an app with selinux on any non-Android non-ChromeOS Linux distro.
In general, non-Android non-ChromeOS Linux is not good at this sort of thing: half a dozen sandboxing frameworks exist, but none of them are particularly secure.
Also, suppose you want to load an obscure kernel module that reads an obscure filesystem format. How do you sandbox the module?
> In general, non-Android non-ChromeOS Linux is not good at this sort of thing: half a dozen sandboxing frameworks exist, but none of them are particularly secure.
There are no frameworks that use secure enclave for this purpose either. It's purpose is copyright protection and preventing user from removing features like advertisement and telemetry, not making your system safer.
> Also, suppose you want to load an obscure kernel module that reads an obscure filesystem format. How do you sandbox the module?
In the actual world with the actual options available, rejecting the technology of the secure enclave has significant opportunity costs. In a theoretical reality in which one of the options available to you and I is a secure-enclave-independent microkernel OS on which you can run a mainstream browser, then you might be right that secure enclaves are unnecessary.
In the actual world, secure enclave is used for DRM, setting user permissions and running untrusted code as another user gets you 80% of the security you need if you don't trust something, and running it in a mostly empty container gets you another 19%. Unless you have a habit of running dubious code that you grant network access and keep up to date to ensure it knows the latest exploits, practically speaking you're fine.
Of course the obvious solution is don't run malware. Android's need for security partly comes from the fact that the primary repository/store distributes tons of dubious code that it grants network access and keeps up to date. If you stick to e.g. F-droid and turn off automatic updates, you don't find yourself in this adversarial position.
>running untrusted code as another user gets you 80% of the security you need if you don't trust something, and running it in a mostly empty container gets you another 19%.
Like I said, the Android team does not think so. Nor does the ChromeOS team, which uses selinux to sandbox the browser, something no other non-Android Linux distro does (except possibly secureblue, which sadly almost no one uses).
