While I get that DMs are capable of completely screwing the pooch here, I've seen enough game designers who were absolutely certain a given rules construct was a good idea given it had worked well under specific conditions that didn't necessarily apply to the majority of people who were going to be using it, I'm not sure the average GM is always worse, at least if they'll take the time to get the overall gestalt of the system before doing it. They at least will usually have a clear idea of the dynamics of the people they're preparing rules for.
I mean, seriously here, the 3e era designers apparently in many cases thought people would play 3e just the way they'd played AD&D2e, even though the mechanics had a very different set of incentives and counterincentives. This is not the sign of people that had a really good holistic grasp of game design.
Let's not forget that 3e and 4e as well, suffered under the "Let's bang out books as fast as humanly possible" syndrome. So, yes, stuff didn't get tested or at least tested rigorously.
And, I do disagree that DM's will usually have a clear idea of dynamics. We all get tied up behind our own confirmation biases and, with a very, very tiny selection and typically very little actual empirical evidence, we will start fiddling about with rules because we just know better. Again, I've seen this (and most certainly done this) fail so, so many times. Heck, I'm in the process right now.
We have an owl folk PC in the group. In the last session, the owl folk PC fell into water. Then took off from water. "Wait," I said, "You can't take off from water."
"Why not?" was the reply. And, in the player's defense, there is absolutely nothing in the rules regarding this that I could find. There is no rule reason why you can't do this.
Now, I could certainly add a house rule for this. Sure. No problem. But, the response was twofold - 1. What does it add to the game? 2. How often is this actually going to come up?
And, frankly, the players are right here. It doesn't really add anything to the game other than make me happy. It certainly doesn't make the player happy and no one else really cares. And, well, it's the first time in a year of play that this has come up and very likely will never come up again. So, what's the point of me making a rule here? What am I trying to achieve?
It reminds me of a DM I used to play with that insisted that my Forge Priest ability to make a magic weapon was WAYYY over powered. He absolutely refused to allow it in the game. The fact that 1. Every cleric in 5e D&D already HAS a magic weapon via cantrips and 2. It would have made a difference in exactly 1 encounter in 11 levels, did nothing to change his mind.
I am very much not convinced that DM's have a "really good holistic grasp of game design". IME, DM's have a very poor grasp of game design.