No, they're not useless at all. The point of shortening certificate periods is that companies complain when they have to put customers on revocation lists, because their customers need ~2 years to update a certificate. If CRLs were useless, nobody would complain about being put on them. If you follow the revocation tickets in ca-compliance bugzilla, this is the norm—not the exception. Nobody wants to revoke certificates because it will break all of their customers. Shortening the validity period means that CAs and users are more prepared for revocation events.

Right. It's the same debate about how long authorization cookies or tokens should last. At one point in time--only one--authentication was performed in a provable enough manner that the certificate was issued. After that--it could be seconds, hours, days, years, or never--that assumption could become invalid.

It did until it got so short that it created a new potential attack surface — the scripts everyone is using to auto update them.

Or that someone asked to renewed it, one of their four bosses didn't sign off the apropriate form, the only person to take that form to whoever does the certs is on a vacation, person issuing certs needs all four of his bosses to sign it off, and one of those bosses has been DOGE-ed and not yet replaced.

expired letsencrypt cert on a raspberrypi at home smells of not paying attention... with governments, there are many, many points of failure.

I am curious how long the approval process in some large corp or the military would be for either of those options...

Hand over our private keys to a third party or run this binary written by some volunteers in some basements who will not sign a support contract with us...

No, it has nothing to do with the time to crack encryption. It's to protect against two things: organizations that still have manual processes in place (making them increasingly infeasible in order to require automatic renewal) and excessively large revocation lists (because you don't need to serve data on the revocation of a now-expired certificate).

It's also a "how much exposure do people have if the private key is compromised?"

Yes, its to make it so that a dedicated effort to break the key has it rotated before someone can impersonate it... its also a question of how big is the historical data window that an attacker has i̶f̶ when someone cracks the key?

No. The sister comment gave the correct answer. It is because nobody checks revocation lists. I promise you there’s nobody out there who can factor a private key out of your certificate in 10, 40, 1000, or even 10,000 days.