Kind of hard to listen when people have, again, failed to provide a counter argument from "You're wrong, I'm right."
And, again, Zero & ZX do a better job with designing levels with the wall climbing & dashing mechanics than the X series.
I have been slowly keeping track of this entire mini-debate inside of this thread. It's amusing how bitter some folks have gotten about it.
Mega Man might be my favorite franchise, so I wanted to comment on this. I gave it a long think, and you're mostly right about the fundamental principals of X's level design not being altered too much. There are large portions of the X games where the dash and wall-grip are almost functionally unnecessary. Tons of segments where level design is essentially a corridor with the occasional barrier to climb over.
Most of the differences are subtle, but nonetheless meaningful. When a player can dash and and pivot like X/Zero can, you have to rethink every hazard. The same goes for the level of environmental intimacy allowed by wall climbing. In the first Mega Man X, the player was radically overpowered compared to fodder enemies. You could dash and pivot effortlessly in the air over vollies of projectiles or cling to walls and poke from a safe distance. However most bosses would find clever ways to effectively disable your enhanced mobility and make you think on your feet. Think Armored Armadillo pinballing around or Storm Eagle's air carrier detonating and him trying to push you off. Plus you had the final swath of novelty bosses in Sigma's fortress with utterly weird properties and lots of vertical thinking. In fact, the coolest thing about Mega Man X versus classic is how much more vertical the level designs tend to be since you have potentially infinite upward mobility.
That being said, very few of these changes are entirely novel. Classic Mega Man had levels where you scaled towers and arenas with death pits. Essentially every trick X games throw at the player has some infant seeds in Wily's Fortress in Mega Man 2, which I think you hinted at more or less with your first post. I would say Mega Man X 1-4 are just refined takes on expanding upon those ideas and giving a player enhanced mobility to conquer them. Mega Man X games are generally far more forgiving and liberal with their option set than classic Mega Man. It's personally why I find them more fun to play on average.
Oh and you're right about later franchises using mobility more effectively. However, in those games, you basically have to use every option at your disposal, lest you get utterly annihilated by even the earliest bosses. Yet, I don't think funneling a player into fixed patterns of movement with low thresholds of failure for achieving pinpoint platforming or conserving specialty ammo is fun. It's why I greatly prefer DOOM 2016 to DOOM Eternal in fact. Totally different means of player expression of empowerment, with radically different thresholds for failure and experimentation. Playing Mega Man X versus Classic Mega Man or Zero is far closer to a joyous romp. Mega Man X is honestly at it's lowest when bosses or hazards kill you in 1-2 hits and stifle how the player feels they can move or how many risks they can take.
Honestly, I am kind of embarrassed there are folks insisting you are wrong simply because you have an outlier opinion. I only disagree that X is somehow worse or less valuable because it doesn't iterate thoroughly. The mechanics are designed to work in an appreciable concert with the expanded economy of options and lower threshold of failure. It's a substantial departure from the classic formula that works for folks for a reason. It's actually great that Mega Man fans often have series preferences because of the minutia of each spin-off.