If an attacker has root privileges to my Linux system, what's stopping them from overwriting my Linux kernel image in my boot partition with one that has backdoors?
It does nothing to prevent overwriting the kernel image.
Now if you ask, how is secure boot stopping an attacker from overwriting libc or some other important system library? the answer is nothing is stopping it on Linux, but on ChromeOS, MacOS, Windows and the two mobile OSes, the secure-boot machinery has been guarding against that for years.