Just wanted to add a few things (speaking as someone with SMF experience)
1. A converter would be great, though I'm not migrating any of my specific SMF installations to XF, so I have no reason to actually write one, nor really the time to do so. Yay for volunteering time. I'm fairly sure that if you paid the SMF devs to work on it full time (like XenForo, IPB, et al), development would be significantly faster, just as I'm sure if someone was paid to contribute a converter, it would happen fairly quickly.
2. Interesting point of note: there actually don't seem to be as many people wanting to leave SMF as wanting to leave MyBB from what I've seen. But maybe my figures are a bit skewed.
3. To correct one point: 'many of the key devs left to work on their own forks' is somewhat incorrect. Only three of the total history of SMF developers left to work on forks, four if you count 'consulting developers'.