you need a Microsoft signed stub to boot anything other than Windows on a PC
False. Every PC I've had has allowed Secure Boot to be turned off, and some of them allow me to add another trusted certificate as well.
you need Apple’s blessing to boot anything on a Mac
False. The Mac boot process is completely unlocked, at least on Intel Macs.
your smartphone manufacturer decides whether you can unlock it and lose attestation
My Pixel 6 allows me to unlock the boot loader at any time.
Attestation exists, unfortunately, but it's not nearly as pervasive as you seem to think.
This is the next logical step, to add “web app” attestation, since the previous ones had barely any pushback
Uh, there was huge pushback. That's why even a Microsoft Surface won't stop you from installing Linux.