Human evolution. Our bodies are not perfect. Yes up till now we're adapted to football well and can play the sport, but gradually our bodies might get weaker and intolerant to that amount of exercise and tension. This is a very early sign. Football in some decades or if lucky in about a hundred years will become nonexistent unless human biological breakthroughs can find the adequate methods to counter it. Otherwise, in few years, football federation will have to cope with our bodily changes by setting new rules, as fewer games in a season, increase field players by one or two and decreasing field length to conserve energy, and eventually decreasing the match length by 20, 30, 40 minutes ...all that gradually in the path leading to football's abolishment.