There's nothing specifically that says S/MIME certs would need to have the same 90-day expiration date, but even if they did, I'm making a basic assumption that

There's nothing specifically that says S/MIME certs would need to have the same 90-day expiration date, but even if they did, I'm making a basic assumption that if there were a standardised, free API to issue S/MIME certs, major email clients would build-in a client to request a certificate - heck it might even prompt major email providers to offer their own solutions for certs, to compete with alternatives that supported using LE certs.