Football is a great spectator sport, almost as good as Christians vs. lions. It has more tactical and strategic depth - probably more than any other physical sport, and certainly more than soccer, basketball, and hockey. Like Christians vs. lions, though, it's very tough on the competitors.
There have been some measures taken - trying to teach proper tackling technique, penalizing hits on so-called defenseless players, and so on, but their effectiveness has been very limited. A major problem is the size and strength of the players. Fast moving 150 kg (330 lb) players are just extremely dangerous.
Limiting substitutions drastically would probably cut the weight of the biggest players by 40 to 70 lbs. Shortening the time between plays by 10 seconds or so would take off another 15 lbs or so. If most players played every down, the really big guys simply could not keep up.
It would also make football teams smaller and cheaper, and coaches less valuable.