why Hoover changed from the metal beater bars to the plastic
This is my own theory, so its subject to being way off, BUT
I think it was because of carpeting trends.
When the beater bar was introduced, everyone had area rugs. The rugs were not glued or tacked down, and it was easy for the cleaner to pick the rug up and clean it "on a cushion of air"
Wall to wall carpeting changed that some, but if the carpeting was flat-er it could still be picked up and "beaten" by the beater bars and cleaned "on a cushion of air"
Look at the performance of a 2-position Convertible on flatter rugs....
Even Kirby used the cushion of air thing in their instruction manuals.
Things changed in the late 60's with the advent of SHAG and glued down indoor-outdoor carpets. Neither rug lent itself very well to being cleaned "on a cushion of air".
Same story with plush carpeting.
Enter the stiffer brush strips on Hoover cleaners starting in 1975, along with the High-Performance Convertible.
Then the Concept One in 1978 with the quadreflex brush. The "beater bars" on that agitator don't really beat, they are more to serve as stiffeners for the brushes. A Concept One is in its element on a thick plush rug. It will clean it better than anything thanks to its aggressive brushing.
Eureka went a different route with their cleaners, via the ESP line in 1979.
Kirby didn't change their brush roll until the Heritage I came out in 1982.
So after all that rambling, what I mean to say is I think they found that an aggressive all bristle design worked better for the modern rugs, and they went with that design.