if i were designing i mine id have it go off straight away with the initial downward pressure.
Your design kills one man.
The fire-on-release trigger delays at least three men, if not the whole squad, making them easy pickings for snipers.
And then it kills the same initial man, as well as probably killing or maiming the two who came to his aid. And if they're maimed, that ties up yet more men getting them medical attention.
War is ugly, but it would be very arrogant indeed to imagine you can out-think a weapons designer.