Bootloaders are not single files. Consider UEFI:
For grub2, there's both a .efi and some configuration that I'll handwave
for purposes of this conversation. For shim, it's more like 4 things -
the main shim*64.efi, fallback.efi, boot.efi, and boot.csv. These all
serve different purposes, and need to get loaded from specific parts of
the ESP. (Recall here that fat32 doesn't have symlinks, either.)
I don't think I understand the problem here. Do shim updates usually change all of
those files at once? What's blocking us from updating them one by one?
Note that bootupd is not trying to solve the "safe" part of bootloader updates:
we're aware that the system should not crash while the bootloader is being updated.
Thus that's why we're requiring interactions from an admin to apply the update as
only they can now when the system is the less likely to crash.
While I think it will surprise no one that I don't agree with
bootupd claims the additional goal of supporting Legacy BIOS. For that,
you also need to consider updates to the MBR, which isn't a file at all.
While we do have that as a goal, we don't support legacy BIOS right now thus the
reason why this proposal is focusing on EFI systems: