I am very skeptical on this point.

macOS, like all Unix systems, already limits privileges for non-root users. What do you accomplish by placing limits on root as well?

If a malicious application gets root, you are very screwed. The app can encrypt most of your hard drive, monitor most keystrokes, do nasty things with your hosts file, and steal most of your personal data. It won't be able to directly inject itself into other processes and certain critical OS files we protected, but how relevant is that?

As I see it, SIP's main purpose is to (1) prevent non-technical users from (completely) hosing their systems by copying and pasting terminal commands from the internet, and (2) to protect TCC.db so that apps can't bypass Apple's privacy system.

If you're able to turn off SIP, you have enough technical knowledge than #1 isn't necessary. I suppose #2 may have some limited value, but not much.

If I am completely off base on this, feel free to educate me—but in my several years of research I have not come across any plausible scenarios for when SIP's protection would be helpful.

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Edit: One other relevant note: Apple lets you selectively disable and enable parts of SIP. So you'd likely be able to turn off sideload-blocking (or whatever it is) without disabling SIP completely, if you want to for whatever reason.