Relinking glibc would block the boot process.

how?

it's a dynamic library, and this isn't windoze with awful mandatory locking

as long as the underlying version is unchanged: there should be no problem whatsoever

glibc is going to get used by everything in userspace, so you’ll need it when you boot.

Yes, but this thread is about doing the linking after boot. It doesn't matter if you link synchronously before you start the program or link asynchronously after you start the program - you will still get a new unique binary for each boot.

yes... it is there at boot

then after boot you relink for next boot