Now you're talking about an airfoil that changes with your speed; you can get that by mixing elevator and throttle. Of course lift changes with airspeed: this is as true of the airfoil you get by putting in some down trim as of any other lifting airfoil. The problem is, as an earlier poster noted, it doesn't change enough to do the job, and the wing's lift changes with speed, too. The wing's bigger. So the reason adding a lifting airfoil wouldn't do what you want (without other changes) is exactly the same as the reason flying with a couple of clicks of down trim wouldn't do what you want. Adding down trim to what was a flat surface is exactly the same thing as adding a lifting airfoil.