I used to be a pretty heavy NNTP user... at some point, while it was largely left to rot, NNTP features themselves became much harder to use... the fact that the leading button on posts now "reply" is an email to the author of a post, instead of a response post is beyond me, and changing the behavior got worse release after release.

The fact that they haven't invested in anything resembling a companion set of services for shared calendar/contacts is also a heavy pain point in contrast to the use of GMail or Outlook/M365/Exchange. If they had offered hosted email/calendar/contacts alone as a monetization option, they could have done so well ahead of GMail or M365 options and could still do so and under-cut them... having an open-core suite just for communications.

They've left a lot of options out there to die... they effectively had Electron a few decades before Electron was a thing. XulRunner was pretty nice to use, and they just left it to die... it got worse over time and just stopped seeing updates. All the while, the charity org and business org just kept spinning their wheels and basically throwing money away... for decades now.