I think you are right that UI sucks in many cases, but I think its not intrinsic to PGP - its fixable.
The UI still sucks, though, because people ask me what the .ASC attachments sent with all of my emails are and if I've been hacked. When I explain that's for encryption, they may ask how to set that up on their phones if they care, but most of them just look at me funny.
I do use email encryption at my job, through S/MIME, and that works fine. Encryption doesn't need terrible UI, but PGP needs support from major apps (including webmail) for it to gain any traction beyond reporting bug bounties.
Encrypted email is near useless. The metadata (subject, participants, etc) is unencrypted, and often as important as the content itself. There are no ephemeral keys, because the protocol doesn't support it (it's crudely bolted on top of SMTP and optionally MIME). Key exchange is manual and a nuisance few will bother with, and only the most dedicated will rotate their keys regularly. It leaves key custody/management to the user: if there was anything good about the cryptocurrency bubble, it's that it proved that this is NOT something you can trust an average person with.
Signed email is also hard to use securely: unless the sender bothered to re-include all relevant metadata in the message body, someone else can just copy-paste the message content and use it out of context (as long as they can fake the sender header). It's also trivial to mount an invisible salamanders attack (the server needs to cooperate).
The golden standard of E2EE UX are Signal, iMessage, and WhatsApp; all the details of signing and encryption are invisible. Anything less is insecure - because if security is optional or difficult, people will gravitate towards the easy path.
The only use-case I have for PGP is verifying the integrity of downloads, but with ubiquitous HTTPS it's just easier to run sha256sum and trust the hash that was published on the website. The chain of trust is more complicated and centralised (involves CAs and browser vendors), but the UX is simpler, and therefore it does a better job.