They fail to realize that there's only so much room at the top of the pyramid, and the useful idiots will only serve as rungs on the ladder for the most vicious who will climb to the top. Thus has it always been with totalitarians.
The rank and file supporters of the Left do indeed think, as you say, that they will have positions of importance in the new communist government. And they may for a short while.
But soon the communist fantasy of utopia will give way to plain old totalitarianism.
The elite realize this, thus the fortified barb wire fencing and the thousands of troops protecting them.