Have you looked at the prices for Z3 M Coupes and 1M Coupes? Those prices aren’t going down so guess who’s price is going up.
2001 BMW Z3 M Coupe, MT for $58,990 with 28,276 miles
2011 BMW 1M Coupe, MT for $58,995 with 5,400 miles
So the real question is, would you buy an M2 for the same price as a brand new M4?
I know your answer is yes, hence the reason why these things are going to be more expensive and stay more expensive than new M4s. If you are the owner of a manual M235i then congratulations, your car will be worth over $30,000 regardless of mileage!