This is a bitcoin forum, so it is guaranteed that (at least) every legendary member has a bitcoin wallet and is capable of signing a message with his private key. Security of that digital signature (ECDSA) is no less than security of PGP signatures (RSA or DSA). But the problem is most members don't have any use for PGP so you would be forcing them to use something they don't need.
Sure, but email clients don't have plugins to decrypt messages encrypted using Bitcoin keys. And the software to sign and decrypt messages in a browser via PGP keys exists, and is mature. No such thing exists for Bitcoin, namely because Bitcoin keys aren't intended for the that purpose.
There's no good reason to be sending unencrypted email in 1999, let alone 2019. Especially a forum concerning leading cryptography tools.
P.S. BIP322 exists for signing standard.
Ok, but it's not yet accepted. We at least need to wait for that, then we have to wait for the majority of Bitcoin wallets to roll that standard out. Meanwhile, all PGP clients can already do the job in a standard way, all that's needed is to avoid relying on fingerprints (which is not so hard). And PGP has interfaces with alot of other types of software.