It's no secret that the new site is having less activity from experienced users than there used to be.
I've seen comments to that effect a number of times, but I'm not entirely convinced it's necessarily true. I do think there's been a drop in over-all activity, but I don't think we have a smaller percentage of experienced users contributing than we used to. We're also seeing increased usage of the blogs, which is probably drawing away some traffic that would have previously been in the forums.
This is all just my own impressions however, and I haven't actually seen any numbers to back this up.
The old layout worked fine for over a decade
Sure, to a certain extent, but the software was a bit hacked-together, and any new features appeared very slowly (if at all) and were sometimes a poor match for the rest of the site. Since the update to the new forum software we've already seen two major feature additions (Classifieds and Marketplace) in a relatively short period of time, and our developers are now free to concentrate on the specifics of this site rather than basic forum functionality, security updates, etc. It has taken some time to settle in to the new system, but I think things are running pretty smoothly now, and we have a solid platform to move forward and really work on making the site the absolute best it can be.
That being said, there are more changes in the works, and I have a feeling that if you preferred the older site layout you might like some of what's coming.