The ball above the shoulder is the one that breaks players down

High-bouncing topspin forces contact in a raised position where the arm has least support and most exposure.

Hitting a ball at hip height is comfortable because the arm is close to the body and the big muscles are working in their strongest range. Force the same player to hit at chin height and everything about the position gets worse at once.

Raise the striking arm above shoulder level and you enter the range where the top of the arm bone sits closest to the bony roof above it. The soft tissue that passes through that gap has less room, and the muscles that hold the shoulder blade in position have to work continuously to keep the space open. Do it once and nothing happens. Do it forty times in a match against a heavy topspin player and the stabilisers tire, the shoulder blade drifts, the space narrows, and the tissue takes contact it was not built to take repeatedly.

The single handed backhand has the worst version of this problem, which is why it is dying, and I do not think the tactical explanations are the whole story. Yes, a high ball to the one hander is hard to control. But the deeper reason players stop hitting it is that the shot at head height asks the shoulder to generate force in a position where it is mechanically weak and structurally exposed, and no amount of gym work moves the anatomy.

Two hands change the geometry rather than the loading. The second arm pushes rather than pulls, so the dominant shoulder is less on its own, but the trunk has to rotate harder to compensate for the shorter reach, and the load migrates into the lower back and the side of the abdomen.

There is a knock-on into footwork, and this is where the injuries actually happen. A player who cannot hit the high ball comfortably retreats to take it after the peak, which means stepping back on a diagonal, which means changing direction while moving backwards. That is one of the least stable movements in the sport, and it is the mechanism behind a fair number of ankle rolls and hip strains that get logged as bad luck.

So when someone tells you the modern game is hard on shoulders because everybody serves big, only half agree. The serve is a controlled effort a player chooses. The chest-high ball on the backhand wing is one the opponent chooses for him, and that is worse.