Complex sentence: Essential complexity is caused by the problem to be solved, and nothing can remove it--if users want a program to do 30 different things, then those 30 things are essential and the program must do those 30 different things.
Split into simpler sentences: Essential complexity is caused by the problem to be solved, and nothing can remove it. If users want a program to do 30 different things, then those 30 things are essential and the program must do those 30 different things.