I have long suspected the big reason most AS anglers went to hairwings had more to due with the fact that it is far easier to learn to tie and to set a hairwing as opposed to a featherwing. I have no proof this was and is still the case today; but I haven't met many tyers over the years who can properly set a featherwing despite their ability to tie hairwings.
I know I read a lot of stuff written by Col. Brooks (and others to be fair) about how a hairwing provides "more movement in the water" than a featherwing; however, when I put a hairwing and a featherwing (including the married wing variety) in moving water side-be-side, the featherwing moves a lot more than the hairwing. Granted a lot of this movement the featherwing displays is sublte; but nonetheless it is moving even when the current is nearly non-existent.
I also read a fair amount from Lee Wulff making similar claims about hairwings; but Wulff also wrote quite a bit about how much more effective his Wulff style dries were in fast water because the wing was stiffer than feathers and as such helped float the fly.
Interestingly, hairwing wets were in use before featherwings for AS with such things as dog hair, horse hair, cat hair, donkey hair, pole cat hair, etc. for the wing. Even Blacker and Francis mention hairwings in their tomes. And in their era, featherwings, especially the married wing variety, were considered the most effective of all flies for AS.
And as I'm sure you know, early PNW steelhead flies were featherwing wets either originals, large trout patterns, or AS flies. And when Glasso's steelhead spey flies became known, they caused quite a stir in steelhead circles because they were featherwings and took a good bit of skill to tie. Likewise, when folks started to use the G.P. tied as originally designed, it caused quite a stir due to its effectiveness.