I notice that the "Popular topics" on the right hand side are usually pretty... stale.
Maybe if the option exists it could be better used to promote "Recently replied to" instead, that way it actually shows what's being talked about.
Seems like on the old forum it was "newest topics" which were better, though still only showed you what had been created recently and not really what was really being talked about currently.
I agree the Popular topics on the right will remain most popular (most viewed) simply because they are listed there.
Unfortunately there is no option to change this. On the left hand panel you can the list of forums and those remain sorted by Pinned at the top and then most recently posted on.
One option you can do is to follow forum area (or simgle post)
1. Navigate to the forum you are interested it.
2. Press the Follow button.
In this case you will get email notifications of posts.
Thanks for the feedback!