Expiries are a defence-in-depth that exist primarily for crypt hygiene, for example to protect from compromised keys. If the private key material is well protected, the risk is very low.

However, an org (particularaly a .mil) not renewing its TLS certs screams of extreme incompetence (which is exactly what expiries are meant to protect you from.)

>screams of extreme incompetence

Not unheard of with the military