This issue is a key reason why I've been desperate to move to Xenforo for literally years. I think people these days (particularly the young) have got used to the latest jQuery type funky UI effects and the ease of use of social networking sites, and in contrast, vB3 feels old and clunky. I hope that the new look AVForums will rejuvenate the community.
It certainly will give it a boost. I've witnessed it myself, and others here have similar stories.
Two important forums I installed it on (one being our "big board", the other being my oldest forum dating back to 1995) saw an increase. With the big board, we were running a customized vB 3.7 that we had tweaked to make more efficient (including using the Sphinx search engine). We had been around 750-800 visitors online at a time, growing slowly but steadily. We were nearly the same in the days following the conversion, but within about four months, we had jumped to nearly 1000 visitors online during peak hours. Summers for us have been slow, but we still get well into the 900s. Posts? We seemed to jump quite quickly from a bit over 7 million to nearly 8 million posts since this past November, when in the past the growth was slower but more steady.
We had our whiners, for certain! Along with the usual members who "hated" the new forum and were going to quit visiting (and of course, came back a few weeks later). Some still take potshots, but for those who ever claimed users would leave us in droves, we have the numbers to show them how much more active the forum has been. Overall, I'd say daily usage is up by about 25%-30%.
Even on my oldest forum, which is low traffic but has a rabidly loyal following, we've seen a similar rise in activity. It also helped to convert since I was running an ancient phpBB 2.0.17 version that had been extensively modified to deal with all the bugs, security hacks and spamming activity, and was therefore impossible to upgrade. Not liking the attitudes on their support forum, I refused to upgrade to phpBB3, and the other free offerings were poor quality also. (MyBB still looks and feels like an amateur work; SMF is essentially dead as far as new development goes.)
Most will welcome the change. A very small percent will complain (quite vocally), but that is normal. Your server will also thank you for it.

It's all good!
