Perfect forward secrecy requires two-way real-time communication, in order to construct a session key that can't be computed from just the private keys and the encrypted message. Therefore the way that PGP's lack of perfect forward secrecy is a feature is that it allows an encrypted message to be generated in a way that doesn't require two-way real-time communication, and can therefore be sent by email.

The trade-off is that you then don't have perfect forward secrecy.

Why does signal still work when the other party is offline?

Seems somewhat like threat model will determine the need for deniability etc. I don't consider myself to need it, and mine seems like a common enough case - compatible with a normal WhatsApp user's use case.