Causes of MPB

[Aston]

See, what you're suggesting is that the inability to grow feathers would have been stressed (instead of the ability to) so that females could identify poor mates, but those genes would never survive, as the ones with them would not have mated and would have died off.

No, according to my speculation the ability to grow feathers would have been stressed, but poor fitness would have caused pea****s to still have no tails or grow smaller tails.
This is what actually occurs to pea****s.
They don't grow tails of the same size across the board. You are assuming that pure sexual selection alone justifies the presence of sexual dimorphisms, but that theory isn't the most popular, in my view.

The Handicap Principle, for instance, supports my speculation, with a famous study proving that pea****s born from males with bigger tails have better survival rates. Thus, tail size correlates to fitness. Females also choose males with bigger tails preferentially. In fact, the pea****'s case is often quoted by the proponents of the Handicap Principle as exemplar: http://en.wikipedia.org/wiki/Handicap_principle

The genetic structure that allows some pea****s to have smaller tails than others is consequently a positive trait, because females can choose the males with the biggest tails and be sure that such males are more fit than the ones with smaller tails. The difference in tail size is influenced by the capacity of the male pea****s to adapt to their environment, which is what we call "fitness". This is even more evident in deer antlers, which serve as ritualistic weapons in male-to-male combat, yet are proportional in size to deer nutrition.

Therefore, if pea**** tail size (only the male birds have such tails) represents fitness, then human males' hair could represent fitness in the same way.

Furthermore, if we consider DHT metabolism, an excess of DHT causes excess body hair and hair loss on the head. Women across the globe have an overwhelming preference for men with a head full of hair and low body hair. (Apparently, only 20% of women preferred men with chest hair, in a cross-country study.)
Increased DHT metabolism thus correlates negatively with reproductive chances, and is typical in old age, which corroborates my speculation further about hair loss being correlated to fitness, or rather, that the presence of hair is a sign of fitness, thus also playing a negative role in its absence, like the smaller tails of male pea****s.

There are always fitter individuals who manifest better traits than less fit individuals and if males evolve ways to "look" fit regardless of actual fitness, females evolve new ways to distinguish fitness in males independently. I believe you may be mistakenly assuming evolution always achieves the ideal case scenario, while it never does so and is in fact a very imprecise, tentative mechanism, which has unarguably doomed countless species to extinction for every currently living one.

As i said before, the recent evolutionary disappearance of energy intake as a fitness indicator (thanks to agriculture) could very well have masked a widespread micronutrient fitness deficit to females, causing them to use a previously positive or minor trait like the absence of hair loss as a major fitness determinant. How? Simply, those with hair loss genes yet no hair loss would indicate fitness and produce statistically better offspring of those without baldness genes. The daughters of the women who preferred men with hair would therefore tend to inherit better genes, survive better and diffuse such trait.

I won't say this theory "must" be correct, but i believe it currently can't be denied with our knowledge of evolutionary biology.