I disagree. I don't think rotating certificates would help against birthday attacks or bad prng.

Tbh, i have no idea which part you are attacking with the birthday attack in this specific context. It doesn't seem particularly relavent.

(At the risk of saying something stupid) - i was under the impression RSA did not use nonces, so i don't see how that is relavent for an rsa cert.

For an ecdsa cert, nonce reuse is pretty catastrophic. I fail to see how short lived certs help since the old certs don't magically disappear, they still exist and can be used in attacks even after being rotated.

Certs should be checked against a CRL and CT for revocation, and expired certs should never be accepted, for this reason among others.

CT isn't used for revocation. CRLs aren't really a thing in practise. Refusing to accept expired certs is important for other reasons but won't save you from a reused ECDSA nonce.

If properly generated even the smallest RSA key sizes used in practice are still safe from birthday collisions.

But there have been several high-profile cases of bad RNGs generating multiple certs with RSA keys that had common factors. I think if you were put at risk by such a broken RNG, frequently re-generating your certs would tend to make things worse, not better.

Don't be nice, or do it thrice - hash input twice.