It's a shame that more kids are not being taught this style. I don't see a reason why this old school game wouldn't still work at the highest levels provided you have all the other attributes of a pro (hand eye coordination, speed, fitness, anticipation, footwork,..etc.)
At the men's pro level you could not hit the ball hard enough with enough margin to be competitive if you hit flat all of the time. Even Delpo can put a lot spin on his fh when he wants to. He only goes for that flat-ish bomb at specific times. Flat all or most of the time would create too many UFEs, and too many situations where it would be physically impossible to hit flat and hard and keep the ball in, so you create a sitter for your opponent.
A lot of the pro women hit pretty flat a lot of the time, so it can work there, though it seems like more women are using more spin.
Rec level is a different thing. That old school style could work well.