Of course there will be drops, but just because demand may flatten and the cars aren't getting any younger. Some people don't necessarily want the bigger Model S for various reasons or want a new car, not a used/CPO.
the cpo comes with a new car warrantee (4yr/50k) and remainder of the original 8yr unlimited on drivetrain. by the time you get to the performance of a cpo you'll end up spending more for the model 3. also the cpo go through rigorous inspections. the condition you get a cpo in is literally almost new. even the tires cant be less than 6/32nds or replaced with new ones.
would you buy a new boxster over a 2 yr old Carrera gts with 20k miles on it with new car warrantee and new condition? I know I wouldn't
the model 3 is going to be a stripped car with no leather, no tech, no supercharger, no pano and so forth. you add all that crap with performance suspension, rims and dual motor option you're looking at 65k easily with only a 200 mile battery.