onetrack,
I couldn't be sure that the owners of PPRuNe are willing to indulge in the necessary upgrades whilst the forum still works reasonably well.
Except the irony is that both vBulletin and PPRuNe share the same parent company. So one would have thought they would have been keen as anything to show off the latest and greatest software.
But as you say, there's a lot of work involved, especially as it's not just a leap between minor but between major versions (3.x.x to 4.x.x). I suspect the current version that runs PPRuNe might have also been subject to a few tweaks and hacks here and there.