Setting Tpc of gate to 0, what prevents me from doing so?
You are combining two true but mutually exclusive ideas.
You certainly can set all of your gate delays to zero and you will have the world's fastest processor, but it will only exist in your simulation.
If you want to manufacture a processor you won't be able to set the gate delays arbitrarily. You will have to use the manufacturer's process parameters to estimate the actual, realizable gate delays...and they won't be zero.