I think 60,000 is the minimum they can have to host NFL. I doubt they'll be too bothered about empty seats given the huge increase in income they'll generate. Plus if they did make it to the promised land I can see them getting 50,000+ for some games, more than double what they get now.
I understand what you mean, but not filling every seat isn't really an issue abroad, it seems we're obsessed with every seat being taken. I would also hope that they'll be many more reasonably priced tickets for games, but that might just be wishful thinking on my part.