Itβs a mental shift similar to using iam assume role sessions rather than static keys with perms right on em.
Powerful creds should be limited in lifespan and machine issued to users in a transaction that involves mfa.
We built our own but Gravitational Teleport I think has these patterns in a product.
The other issue even if you get certs right is service tokens (an admin could have grabbed a copy of one and they can be used from outside cluster) so want to keep that api wrapped IMHO.
As an aside, if you want revocable, non expiring user creds you would be better served by bearer tokens.
That said the distributions I've seen that make use of client certs, don't do that (typical lifetime for a client cert is 1 year), so I'm guessing a load of people using k8s will have these certs floating about...