That sounds quite logical except... how do you explain the sale of 3x E39 M5s in the span of 2 weeks, about a month ago. One for $40k, the others for more than $50k? Each of the sold in less than two days. I know because I bought the 3rd one on the day it was advertised, after missing the 2nd one by waiting one day to call. The 1st one ($40k) was sold by a foolish person who set the buy now $5k too low. There is still one for sale now but, as with these M3s, it is overpriced ($65k, high kms). There have also been 2 or 3 Evo M3s sold in the last few months, though each of them took a few weeks to sell. There is money out there, but it's not foolish. The Evos took time to sell because they were price just above market value. The only E46 M3s I've seen sold are top condition examples for acceptable prices ($55k).
But, to support your argument a bit, E39s are 'old man' cars and they (we!) are perhaps more likely to have spare cash. M3s are 'younger man' cars, perhaps less cash lying around?