Not as if there are any numbers around, but I'd reckon that the number of people who would be injured by the fence (a stationary obstacle, mind you) would probably be about the same as the number injured diving at the rope, or even just by throwing themselves about elsewhere in the field anyway. There will also be some extra distance to allow people to take diving catches—parallel to the fence—of balls that reach catchable height in the space currently outside the boundary.
Of course, it's not just fences; at English Grounds, we should return to the the old fashioned painted line with the crowd sitting just behind it completely exposed.
And speaking of painted lines, I remember that years ago, the rope—at Australian Grounds anyway—always had a painted line underneath it, and for some reason this seems to have stopped being done; given that it allows the actual position of the boundary to remain obvious if the rope moves, why did this stop?
Anyway, I'd also restore the old fences and scoreboards (where applicable) at Australian Grounds (and maybe de-develop Adelaide to the old shape whilst that's happening).