This maximizes the net benefits of the activity. The principle of substitution decribes what happens when optimal decisions have been made and then something changes. A rise in the marginal cost leads individuals to substitute alternatives for the activity whose cost has risen. A rise in the marginal benefit leads individuals to substitute the activity whose benefit has risen for other activities. Second-round effects implies that the initial response to a change is seldom the last action.
Markets respond to changes with changes of their own.
The final outcome might be very different.