What I don't understand is why GRUB needs to be regenerated every time you add a kernel or OS.

Make the bootloader smart enough to find read the underlying filesystems and find kernels. rEFInd does it. If I put in a bootable USB or install Windows on a second SSD, it just shows up on the menu without explicitly demanding it.

Maybe you have to say "/boot needs to be one of n well-supported filesystem types" but one-size-fits-most is probably good enough, relying on something GRUB for the people running ZFS-atop-RAID6-atop-a-collection-of-128-USB-floppy-drives.