Football's a rural and suburban game; I'd be willing to bet that a huge minority of SU ticketholders are city residents. Don't think those numbers have much to do with the attendance.
Though I helped start this argument, I don't think population changes (or lack thereof) are what we should look at.
Aside from the persistent losing, it's just a TV problem.
Going to the Dome in the 1990s >>>>>>>>>>>>>>>>>>>>>either not seeing a blacked-out game or watching it on your 25" cathode ray tube TV.
Watching every game on a 60" HD screen in 2017>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>going to the Dome. (At least for a lot of people.)
I don't think we're going back to the old days on this one. Especially now that a new generation of people has been raised with no clue that the Dome can be a fun place to watch a football game with other fans.