The title of the post and the actual question aren't necessarily the same thing. Attendances this season, including Wembley have been very good considering that the vast majority of fans are having to be exceptionally careful with their budgets. For the price of a match day ticket, you could get a decent weekly shop from Aldi, and unfortunately for some, they can't afford both.
I don't think we're in any sort of fanbase decline, in general. If the point is that ticket sales could go drastically down, I'd argue that following an utterly embarrassing season which involved fans walking out half an hour into matches, I've been massively surprised at the ticket sales this season right from the start. I fully expect attendances to be the same or improve next season given it's another year in a league where we compete brilliantly in terms of results. I think if Wembley showed anything, it's that there's plenty of people out there that want to watch Sheffield United in person, but are only able to if there's a way to bypass the points system so they actually can purchase a ticket.
As others have said though, future threats to match day attendances all revolve around the price of tickets, the cost of living, Sky schedules and IPTV.