Here’s what I think: if they actually do everything with open standards and PGP by the book, why can’t they provide IMAP/SMTP access to everyone who wants it BUT add the disclaimer that you’ve to use a PGP compatible e-mail client and configure it to deal with the encryption… they could even configure their submission to refuse any email that isn’t PGP encrypted to improve things further. The fact that they don’t do this leads me to believe that they either a) aren’t actually doing everything as “by the book PGP” and there might be security issues or b) they’re “privacy” as a catch all excuse in order to push a bit of vendor lock-in.
Their market niche is privacy conscientious people and those same people tend be to computer savvy and I bet half of them would mind setting up PGP on Thunderbird and use Proton without a bridge. Everyone else could still use their apps, web or the bridge.