Earlier quoted context omitted.
The Signal protocol is very well designed but the implementation requires a telephone number (I know it is coming). That's a step removed from PGP which can be completely offline.
That's interesting and all, but my point is just that you'll have to stop and think about how many orders of magnitude separate the userbase sizes. A simpler way to make the same point would be that relative to modern cryptography, to a first approximation, nobody uses PGP.
While Debian and other package signing continues to use PGP (and yes, Debian may move away, and signify/minisign is preferable) there are lots of “infrastructure” uses.