I feel like people could take an advantage of a feature introduced 15 years ago and avoid update-grub, complicated scripts, gain faster boot speeds, easier kernel installation, boot management in firmware, etc. Not sure what you mean about “one class of machine”, but x86_64 is 98% of Ubuntu installs.
Signed kernel is a little bit different to a signed .efi package of kernel+initramfs+config as a whole which is loadable by UEFI itself without grub in between. This isn’t a mandatory feature, config file would be enough.
There is a good presentation and explanation of efi/grub situation in the first link in my original post.