The problems with PKI/PKIX all go back to terrible, awful, no good, very bad ideas about naming that people in the OSI/European world had in the 80s -- the whole x.400/x.500 naming style where they expected people to use something like street addresses as digital names. DNS already existed, but it seems almost like those folks didn't get the memo, or didn't like it.
What people forget is that you do not have to use the whole set of schema attributes.
And encrypted and/or signed email? That too, though very poorly, but the issue there is key management, and DAP/LDAP don't help because in the age of spam public directories are not a thing. Right now the best option for cryptographic security for email is hop-bby-hop encryption using DANE for authentication in SMTP, with headers for requesting this, and headers for indicating whether received email transited with cryptographic protection all the way from sender to recipient.
As for the "ability to address groups in flexible ways", I'm not sure what that means, but I've never see group distribution lists not be sufficient.
As for group addressing, distribution lists are pitiful in comparison especially on discovery side.
Anyway, ultimately the big issue is that the DAP schema is always presented as "oh you need all the details", when... you don't. And we never got to point of really implementing things well outside the more expected use case where people do not, actually, use them directly but pick by name/function from directory.
Oh I can't remember. Binary attachments have worked since I started using them long long ago. It worked at least in the mind-90s. Back then I was using both, Internet email and x.400 (HP OpenMail!), and x.400 was a massive pain (for me especially since I was one of the people who maintained a gateway between the two). I know what you're referring to: it took a long time for email to get "8-bit clean" / MIME because of the way SMTP works, but MIME was very much a thing by the mid-90s.
So it took a while if you count the days of UUCP email -- round it to two decades. But "by the md-90s" was plenty good enough because that's when the Internet revolution hit big companies. Lack of binary attachments wasn't something that held back Internet adoption. As far as the public and corps are concerned the Internet only became a thing circa 1994 anyways.
> As for group addressing, distribution lists are pitiful in comparison especially on discovery side.
Discovery, meaning directories. Those are nice inside corporate networks, which is where you need this functionality, so I agree, and yes people use Exchange / Exchange 365 / Outlook for this sort of thing, though even mutt can do LDAP-based discovery (poorly, but yes). Outside corporate networks directories are only useful within academia and governments / government labs. Outside all of that no one wants directories because they would only encourage the spammers.
And in some ways desire of people to send non-7bit-ascii text as email is also a continued brokenness in SMTP email.
As for directories - my point was more that directories hid the addressing details from surface UI. Otherwise AFAIK X.400 works perfectly fine without using everything in the possible schema.
Fun fact - Exchange is actually X.400 system, despite no longer having non-SMTP connection options. But its internals are even wonkier, like Exchange and Outlook not supporting HTML Email (no, really, MAPI.DLL crashes on HTML email. If you send/receive HTML email it's stored as HTML wrapped in RTF, and unwrapped when sent elsewhere)