By contrast, a client-side cooldown doesn't require very much ecosystem or index coordination.
This kind of thinking is why I don't trust the security of open source software. Industry standard security practices don't get implemented because no one is being paid to actually care and they are disconnected from the users due to not making income from them.
(With that said, I think it also varies by ecosystem. These days, I think I can reasonably assert that Python has extended significant effort to stay ahead of the curve, in part because the open source community around Python has been so willing to adopt changes to their security posture.)
There's risk there of a monoculture categorically missing some threats if everyone is using the same scanners. But I still think that approach is basically pro-social even if it involves a "cooldown".
Exceptions to quarantine rules just invites attackers to mark malicious updates as security patches.
If every kind of breakage, including security bugs, results in a 2-3 hour wait to ship the fix, maybe that would teach folks to be more careful with their release process. Public software releases really should not be a thing to automate away; there needs to be a human pushing the button, ideally attested with a hardware security key.