I agree with this, I do not believe that the new forum will attract more users just because of the new design and some new features. For me personally, the forum as it is now serves its purpose quite well, and I like it more than the new version.
I think the main reason for user decreasing is BTC price, but also fact that from 2017 until today, thousands of users get permanently banned. It's also not easy to create new high ranked accounts due to merit system. All this together is the reason that there are currently no more active users, but I'm sure the next big bull run will activate them again.