If you fall, supposedly the gradient's steeper and your more likely to cause an avalanche effect. Than falling over would on a terrace.
The argument should be - Theres terracing in championship and lower league grounds, people dont die, or are you suggesting that premiership fans are too dense to stand up for a period of time.
Good stewarding and crowd control prevents injuries, not the watching position.
Me Id prefer to sit but i quite like standing at some games. It means if theres an irritating racist tit stood near you, you can move away and point him out ot the coppers