Sanguine Do you mean what happened to the traffic? That was just neglect over the space of years. That plus Facebook groups made it easy for folks to switch.
As for the migration, writing a tool to do the migration took a LOT of work. I built it in Laravel so I could piggy-back off ALL of its goodness. Definitely won't be releasing it for public consumption though. Maintenance will be too much work. I respect anyone who does this and then still releases it for others to use.
If I had to do it over, I'd use PHP 7.0 because the Facebook social login doesn't work with 7.2. I think that's when certain encryption stuff was deprecated.
There are lots of things I'd add but just don't have the time. I started writing a PM module but never made it very far there.
At least one member has been vocal about hating the lack of pages. Honestly, I'm not bothered. Some folk want to keep things how it worked 10 or 20 years ago. I'm not worried about them.
The fact that the forum had almost died by the time I converted made it much easier. Much less people to disturb.
One thing I think I did that was clever is I kept the old database around and made a Flarum extension that checks the login against the old DB. If it works, it saves it in the Flarum DB. That way, people didn't need to change their passwords. That and Flarum's "forgot password" feature means most old members can get back on if they can remember their login or if they still have the same email.
Also, I use AWS for sending emails. Having your "forgot password" or "someone replied" emails not be delivered is unacceptable to me. So I'll never do it any other way. I believe that and the weekly newsletter we run is helping to get back some of the old members.
Hope this helps.