Linux Foundation to host Let's Encrypt

Why not?

UEFI Secure Boot would require them to sign only a handful of bootloaders and then secure the initial keys.

Global, free, SSL certificates requires massive ongoing infrastructure, constant security audits, and may well be a victim of its own success.

Certainly there may be costs involved in being a UEFI root, but do they really compare to running a global CA offering free certs, even via an automated always-online system? You could sign one UEFI bootloader and then never sign another ever again, if necessary.

I think they didn't become UEFI root because they knew they could never fulfill the other side of it (business - convincing manufacturers to put their roots into the BIOS of every computer), not because of infrastructure costs.

But the infrastructure costs of an automated, global, free SSL CA must be humungous, especially if you're intending to verify every domain and sign every cert automatically and include it in an "apt-get" manner in distros.