How is an infection hidden somewhere in the friggin entire rootfs easier to detect and remove that one that literally replaces the one file for your kernel /boot ? What advantage could the latter possibly have ? Not to mention that something from a bootkit bootstrapping an infection in the root filesystem is the realm of useless tech demos like this one; while for something that can already write your rootfs, infecting the kernel is trivial.
The entire boot system has much, much fewer places for malware to hide compared to the entire "rootkit" OS attack surface which is astronomically larger. Secure Boot has always targeted the smaller and most useless of the swiss cheese holes.
Bootkits are to make the infection both more difficult to detect and remove, so whether /etc/init.d is writable is pretty irrelevant.