If you define untrusted software as anything not coming from your distro's official package manager, many people run lots of untrusted software on their Linux systems.
Think of PPAs (Debian/Ubuntu/etc), the AUR (Arch/Manjaro), and many other package managers (pip, conda, npm, cargo, etc.).
You have always the option to put programs that you don't trust into a jail. It just shouldn't be mandatory.
Besides, the only good way to solve that problem is namespaces as they are used in Plan9. Sadly Unix/Linux took the wrong turn at some point (probably with the introduction of sockets).
