This is a very x86 PC-centric view. It is not 'how Linux works' in a more general sense. Other hardware boots in various other ways, not involving BIOS or GRUB, and may or may not have an initrd.
An initrd slows things down quite significantly which is a big deal for some use cases. It provides great advantages in terms of single-boot-image generality and ease-of-rescue but also a cost in speed (and size, although that's small these days at ~2-10 Mb compressed). Boot speed and image generality tend to be in direct competition.
And to answer another post further up, plenty of 'fairly embedded' machines provide a (serial or USB or LCD) console though which rescue can be done and you don't simply want to re-flash and (for example) lose 3 years of logging data. I am thinking of the balloon board controlling my heating system in this case.
Whether overall, moving everything to /usr is a good plan remains to be seen. Personally I am skeptical but I have only spent a limited amount of time reading about the implications so far.