The only reason the TV money stops coming in is if the interest in the sport in England collapses, and if that happens attendance will also go down as you'll be on a vicious circle no investment, best players going somewhere else etc etc, game retreating to its traditional hardcore fanbase.
There won't be a scenario where there is a huge cut in TV revenue while at the same time 50,000 people are going to regular league matches. If interest in the sport falls away that much, then any newly expanded stadiums will start to look like white elephants, not insurance policies for the future.
There was no Sky money in the eighties and even the big clubs were scraping 20-30k through the door.