I won't enter into all the details but... It's totally possible to not have the sudo command (or similar) on a system at all and to have su with the setuid bit off.
On my main desktop there's no sudo command there are zero binaries with the setuid bit set.
The only way to get root involves an "out-of-band" access, from another computer, that is not on the regular network [1].
This setup as worked for me since years. And years. And I very rarely need to be root on my desktop. When I do, I just use my out-of-band connection (from a tiny laptop whose only purpose is to perform root operations on my desktop).
For example today: I logged in as root blocked the three modules with the "dirty page" mitigation suggested by the person who reported the exploit.
You're not faking sudo with a mocking-bird on my machine. You're not using "su" from a regular user account. No userns either (no "insmod", no nothing).
Note that it's still possible to have several non-root users logged in as once: but from one user account, you cannot log in as another. You can however switch to TTY2, TTY3, etc. and log in as another user. And the whole XKCD about "get local account, get everything of importance", ain't valid either in my case.
I'm not saying it's perfect but it's not as simple as "get a local shell, wait until user enters 'sudo', get root". No sudo, no su.
It's brutally simple.
And, the best of all, it's a fully usable desktop: I'm using such a setup since years (I've also got servers, including at home, with Proxmox and VMs etc., but that's another topic).
