I don't think they should, especially since sending and receiving PGP encrypted emails is easy when you use a non-web mail client.
IMO, it's ultimately a problem with Gmail - which makes integration with PGP hard - and not a problem with PGP emails itself. If I use Thunderbird, or even Mail.app, integrating PGP is as easy as installing a plugin, after which sending or receiving a PGP encrypted email requires a minimum of additional effort (once you have a key in your keyring, it's encrypted by default).
If Gmail made PGP integration easy, there would be no extra time wasted when viewing PGP encrypted emails. The validity signaling would even remain the same, but without the "this is hard for me so don't do it, even though we invite you to do it" idiocy.
IMO, it's ultimately a problem with Gmail - which makes integration with PGP hard - and not a problem with PGP emails itself. If I use Thunderbird, or even Mail.app, integrating PGP is as easy as installing a plugin, after which sending or receiving a PGP encrypted email requires a minimum of additional effort (once you have a key in your keyring, it's encrypted by default).
If Gmail made PGP integration easy, there would be no extra time wasted when viewing PGP encrypted emails. The validity signaling would even remain the same, but without the "this is hard for me so don't do it, even though we invite you to do it" idiocy.