[xg15]

But that's just semantics. GP's point was that with Must-Staple, the "real" expiration period becomes pretty much irrelevant - instead, the lifetime of the OCSP response becomes the new effective lifetime of the certificate.

If you compare (1) a short-lived certificate and (2) a long-lived certificate with Must-Staple and short-lived OCSP responses, the benefits, security properties and failure modes of both are exactly the same*. You're just putting the timestamp into different fields.

(* Or almost: Some notable practical differences are described in the sibling comments - but those are mostly a property of LE's current policies, not the protocol itself)