Thanks a lot to everyone for their patience and for re-learning some new muscle-memory habits.
I still haven’t had to time to write a follow-up to my initial post that explained the justification and necessity for this move, but suffice to say that there were definitely some important reasons for doing so.
I would say that the usability and discoverability of the old forum UI was marginally good, but not ideal. As proof of that, I would receive multiple PMs every month from new users asking “How do I post a new thread?”. Of course not everything is perfect here yet either, but it’s important to differentiate between usability and familiarity. And as for functionality, Discourse has the old Drupal platform beat hands-down, at least as far as forum functionality goes. Its functionality does apparently need to be made a lot more discoverable though.