Summary’s not counting private messages. I’ve been using it to ferry progress pictures in PMs. Check your inbox. Ia! Ia! Picturespam fhtagn! <- Spoiler blurred because I was too slow. Tarot said it first… which I saw happen in real-time, with the post appearing while I was writing this reply. Saw him edit it, too. 's more like an instant messenger than a forum.
So, yeah, I can vouch for the existence of a minimum of one person that would prefer to use the new forum but continues(god this editing is so smooth) to primarily post in the old one, for no reason besides that there is more going on in the old forum. I rather expect there are others in the same position.
Taking your train of thought for an example, I think maybe we can see why some of the other people didn’t make the switch. Adjusting to a new environment takes time and effort. The judgement call that it’s not reasonable to spend time and effort learning to use a different kind of software when you’ve alredy got something that’s working just fine is perfectly reasonable.
The new forum really does have a pile of extremely useful features, though. Sudden change’s uncomfortable, but the benefits here are pretty damn amazing. And, the site address doesn’t change. There’d arguably be less splitting up of the community into different forums, since we’d have only a single forum once again.
edit2: … wait did I just see there’s an “Invite Others” button on private messages?!