I might be confident that bad guys won't realise what's going on and somehow break into my email system replacing your public key with their own in, say, six hours it takes to receive and act on the message; whereas I can't be reasonably confident that all my mail systems and all backups of mail systems will remain unbreakable for the foreseeable future or until I've deleted all trace of the symmetric key.

Cryptography is often not about "uncrackable" but about confidence in security over a foreseen interval.

On the 4th of June 1943 the details of Operation Neptune ("D-Day") were a priceless secret of the Allied Command, a break that allowed Berlin to know the plan could have resulted in the invasion force being expected by a heavily prepared and well-reinforced German defence. Those who made it back to England alive would have no reward for their efforts.

By the 7th of June 1943 plans for Neptune were a historical curiosity of little military value.