The Sphere spell is quite different from the Wall in many ways. One of them being that the Sphere isn't air tight. Or is it? Does it act as a filter, so poison gasses are kept out but breathing isn't impaired? Does it offer some sort of life support instead?
We know that a "targeted Dispel Magic" can take down the Sphere, but not the Wall. But the Wall never says, or even suggests that it it won't stop a Gust of Wind. Quite the opposite. And a window screen allows normal breathing, but definitely changes the air flow.
So, meaning no offense, I think your analogy is flawed.
The Save v the Wall isn't like a fall: There's no outside force driving the Dragon into the wall, just inertia which can be reversed or redirected. There is an outside force not simply driving a falling creature into the ground, it's accelerating the creature towards the ground.
Also, the ground is a lot larger than the Wall spell. Much harder to miss.
Now, as I've mentioned a few times, the original question specifically mentioned RAW. That's not the same as RAI, or how any particular table plays the game. My answers were specific to RAW. I wasn't trying to tell anyone how to run their game, and strict RAW isn't how I run mine.
One problem I've found with trying to convert speed into falling distance is that D&D falling rules are linear: Fixed increases in damage based on distance. But falling speed is non-linear, in terms of distance. It's linear relative to time, but as falling speed increases it takes less and less time to cover a given distance (such as 10 feet). Turning game movement into falling distance gives very unsatisfying, uneven results. You double the damage at less than double the speed.
Add in the in-game time spent doing the Trig' involved will tend to kill the adrenaline of the moment. A serious action sequence, with a three minute interlude for calculating the acceleration curve and plotting it against a distance, will kill the fun far faster than it can kill any Dragon ever hatched.