If cap-and-trade held the same levels forever you would be correct. But all of the cap-and-trade systems I am aware of have a built-in lowering of the cap over time. So they start out doing nothing/very little, then ramp up to meaningful reductions over time.

The idea being both to make it easy to get people to agree today (the reductions are tomorrow's problem), and to allow time (and foresight) for industry to adapt to where things are going.