The big problem is people have become addicted to their polyester strings for spin. These strings are so low powered that it drove manufacturers to power up their frames with stiffness for energy transfer and lightness for RHS. That was a recipe for arm disaster amongst the masses.
The secret is to make the frame flexible enough to tolerate Poly strings but powerful enough to overcome polyesters intrinsic lack of power.
My solution has been to gravitate to Prince Frames and use Gut/poly hybrids at low tensions. But Wilson may have solved the conundrum with a flexible racket that is meant for low power poly strings. Time will tell if they've done it. Reviews are all over the place right now.
But getting power, spin, control and comfort together in one package is the dream of all players and racket makers. Right now everything is a compromise and you choose based on priorities.
I know what you mean, but I always had the impression that it was sort of the other way around. Light and stiff racquets were being churned out more and more, but once the word got out about poly strings allowing players to "buy more control", that's when the popularity of those strings really exploded. No biggie - I think that the two trends have gone hand in hand in a sort of co-dependence.
I stuck with my favorite syn. guts when the whole poly thing was taking off. A pal of mine who was in the string business back then quickly warned me off of the polys when I asked about them. He had already been through that full blown case of tennis elbow that we all fear when he tried the string in his own racquets. He gave me a good deal of insight on the general properties of the polys including his cautions so that I'd be careful with them in the future as I got into stringing more racquets myself.
I think I read about poly strings being introduced a long time ago (maybe the late 80's or early 90's?), but they didn't take off immediately because not many players could take advantage of them yet. They seemed to take off as the gear got lighter and stiffer as well as swing technique moving in a more modernized direction.
Yaddah-yaddah... I'd bet that softer racquets could find a solid place in the market. The caveman clubs of yesteryear may be pretty much gone, but it's still the case that frames with decent heft can readily produce powerful shots. Extra flex in a racquet gives me more control, so a combo of moderate flex and "enough" heft to be stable through the ball seems to have no downside. Especially considering that I don't need to string with poly looking for more control - so that checks the arm-friendly box, too.