What if you take all the useless boards and put them into one board? Just make all the useless boards subboards of that board.
How about we do what most forums do and make a "Trash" board?
This "The Void" #OrangeCosmos
Already exists. That is where all deleted threads and posts go.
Its a standard thing in any SMF forums.
Anyway, why would you even clean up these boards? They aren't in the way that badly...
Most, if not all active boards are all on the top of the main page, so they shouldn't be that much in the way. And like Somone said: They are filled with google search tags, so a good way for new users.