It's totally crazy that we have to go through Microsoft to sign things to be able to have our OS run on third parties computers, and that Microsoft manage to win about this so easily as it was never seriously challenged.
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AFAICT you can still disable Secure Boot in most UEFI firmware, and boot anything you like (or not like, if an attacker tampers with your system).
"attacker tampers with your system" does not happen at least in the way you think it does or it does not protect you against meaningful attack at all.
On Windows, secure boot has worked pretty well when it comes to rootkits. MBR rootkits were trivial to write, but UEFI rootkits require UEFI firmware changes or exploiting the bootloader process itself, both of which are much more complex. If malware uses the Linux shim, the TPM will notice and refuse to provide the Bitlocker key, so your computer won't boot without going to the IT office and asking for the recovery key (which should prompt more investigation).
Boot sector viruses, or their modern equivalents. Basically, anything which injects itself into the boot chain before the antivirus can start; after that point, the antivirus is supposed to be able to stop any malware. That is, they wanted to prevent malware from being able to hide from the antivirus by loading before it.
The more realistic scenario would be exploiting a privilege escalation bug. Of which there have been and will be plenty of on both Windows and Linux.
Honestly I wish they(where they is them that designed this whole broken system) did it it right. On first boot you would set up some keys, now you are your own trust root, and when you you want Microsoft to manage your system, perfectly reasonable, managing systems is scary, you sign their keys and add them to the store. The problem is at a low level it all sort of just works, but nobody want to design that user interface. nobody wants to write the documentation required to explain it to joe random user. Nobody wants to run the call center dealing 24/7 walking people through a complicated process, patiently getting them unstuck when they loose their keys, explaining what a trust root is and why they now have to jump through hoops to set one up.
I like to believe that had they done it right initially, the ui would have been molded into something that just works and the client base would also get molded into expecting these key generations steps. But I am also an optimist, so perhaps not and it is exactly as scary and thankless a task as I described above. But we will never know, Microsoft took the easy way out, said we will hold the keys. And now you are a serf on your own machine. Theoretically there is a method to install your own keys, and it may even work, but the process is awkward(never really being meant for mass use) and you are dependent on the vendor to care enough to enable it. Many don't.
The firmware validates the shim. The shim validates the boot loader. The boot loader validates the kernel. The kernel validates the kernel modules.
Once you have that chain of trust, you can also add in other factors; encrypt your disk using a key, seal the key in the TPM, and lock that key behind validation of the firmware and the boot loader. Your system boots, those different components are measured into the PCRs, and if the combination is correct the key is released and your disk can be decrypted automatically. Now if someone boots your system using a different firmware or boot loader, the TPM won't release the key, and your disk can't be decrypted except by someone with the passphrase, recovery key, etc.
Without secure boot, you can't trust that any of those components aren't reporting falsified measurements to the PCRs, lying to the TPM, and getting access to the key to decyrpt your disk despite booting from a compromised USB drive. That, of course, means you can just encrypt your disk using only a passphrase that you manually enter, but for a lot of users (sadly) that's too complex and they'll choose not to use disk encryption at all.
Case in point, TouchID and FaceID are seen as alternatives to using a PIN or passphrase to unlock your iPhone, but they're actually meant as alternatives to not locking your phone at all - a way to make device security transparent enough that everyone will use it. Without a secure chain of trust from the firmware to the kernel, that's not really an option.