Even then, is the message "stop using chrome after this date because half the internet will break" (because where will all those non-paying people go to?), or "stop using LE and start paying someone for a free service"?

I bet google themselves would be scared of anti-trust lawsuits over this. Even if they weren't, i don't think they'll really go so far as to compromise the security of half of the internet just to get their way on this one small improvement.

The point about antitrust lawsuits I concur, but LE is not the only free-as-in-beer ACME. For one, there's ZeroSSL, then Actalis, SSL.com. For some time BuyPass offered free certs, but it does no longer. Last but not least Google itself has Public CA that offers certs over ACME, a fact that I think would be a huge fulcrum for antitrust suit. I would also expect that all other CAs would deploy ACME endpoints to attract at least some part of the cake (note they're in business of being vultures already). So the message will be „go find another CA, here are three examples, sorted randomly like the European first boot UX, just change the URI in certbot config".

Perhaps this shouldn't be left to the CA/B board, it has critical economic impact on many countries, it should be regulated by them?

Either way, I think LE has enough power to at least push-back and see where things fall. continuing to support users can't hurt them, until they truly have no other choice.

> [...] it has critical economic impact on many countries, it should be regulated by them?

This was exactly the point of recent (2024) eIDAS update, which introduced EU Trusted Lists. The original draft was that the browsers were mandated to accept X.509 certs from CAs („TSP”s) accredited in EU by national bodies. Browsers were supposed not to be free to just eject CAs from their root programs for any reason or no reason at all, but in case of infractions they were supposed to report to CAB or NAB that would make the final decision.

Browesers responded by lobbying, because the proposal also contained some questionable stuff like mandatory EV UI, which the browsers rightfully deprecated, and also it wasn't clear if they can use OneCRL and similar alternative revocation schemes for mitigations of ongoing attacks. The language was diluted.

Interestingly though, doesn't this threat become less credible the shorter certificate lifetimes get? Back in the day they could just do this and server admins would figure out how to switch to a new CA the next time they got around to renewing their certificate. Now though that's all automated, so killing a CA will likely nuke a bunch of sites.

This is good point. I think it would still be discounted in favour of suggesting another CAs that users can switch to, but you're right, the promise was that cert management would be hands off, and changing CAs is not hands off in any ACME client that I know of. Best Google could do would be to shift the blame to LE/ISRG, because it was ISRG that promised this automation.

They can do this with certificate transparency other wise CA can sign whatever date they want. But if they collude with CT that can issue rouge certificates for targeted attacks.

Yes, that's all right, there's already a requirement that they submit to one Google CT log and one non-Google CT log. They thought about it already. The playbook I mentioned they've been rehearsing contains specific threat against backdating certs, they say they'll distrust immediately if they detect, and they have means of detecting backdating on significant scale (esp. for LE, where they submit 100% issued certs, not just the subset that is intended for consumption with Chrome).