It's not just that people are using webmail that's preventing PGP adoption - it's that PGP, specifically, is orthogonal to usability and convenience of email.
I, for example, am capable of encrypting my email but I actively don't care to. I'm in the "I don't, because the content of my email is just not that important." on the poll.
Webmail's prevalence may make PGP adoption more difficult, but I want more webmail prevalence and I don't care about PGP adoption and I'm pretty confident that's the way the world's going to go. Someday there will be something webmail-like which has encryption anyway and that might catch on - I'd even use it. In the meantime? People who need it can use special tools.
You may not think that your email's that important but your unwillingness to care means you've inadvertently extended that judgement to everyone else who may want to correspond with you. This effect worries me the most about our modern, connected world.
NB: Just to be absolutely clear, I'm not picking on you specifically (and I suspect there are many people who hold the same opinion as you). I'm just trying to point out what I see as a problem with the "my email isn't important" argument.
I don't really see email as any more secure than snail mail right now (which can be taken from your mailbox and read, albeit with stiff legal threats if anyone catches you). There wasn't an appropriate poll option for "I don't care because I don't think email can be both effective and secure in the near future and I prefer effective".
People who need encrypted email will figure out how to do it and I think that's enough for now. I don't think lamenting how slow PGP adoption is makes sense, though - it will never, in its current form, be mainstream.
I personally consider webmail to be way inferior to any decent standalone client. I only use it out of necessity, when no other option is available. Thudnerbird with Enigmail and IMAP+SMTP do their job just fine otherwise. But the fact that many people have no clue about encryption makes it harder to use.
I don't think PGP is complicated. I think the implementations are subpar (mainly due to lack of interest toward encryption and thus "good enough" generally won)
Even the command line gpg options aren't quite right..
I, for example, am capable of encrypting my email but I actively don't care to. I'm in the "I don't, because the content of my email is just not that important." on the poll.
Webmail's prevalence may make PGP adoption more difficult, but I want more webmail prevalence and I don't care about PGP adoption and I'm pretty confident that's the way the world's going to go. Someday there will be something webmail-like which has encryption anyway and that might catch on - I'd even use it. In the meantime? People who need it can use special tools.