Let's Encrypt is a single point of failure.

WebPKI also suffers from an inability to properly do delegation. It's not possible for me to create an intermediary certificate valid only for *.mycompany.com

If I want to use WebPKI, I have to either expose every host inside my company to everyone (via CT transparency logs) or use a wildcard certificate. And wildcard certs allow attackers to impersonate anything within my domain, if they get access to just one host.

X.509 technically supports name constraints ( https://www.rfc-editor.org/rfc/rfc5280#section-4.2.1.10 ), but its implementation was inconsistent. In particular, some implementations did not apply it to the Common Name. Fortunately, Common Name is on the path to deprecation.