Oh this is so wrong, leveling dungeons scaling can easily be abused with the right gear, scaling it nowhere near as advanced as you are saying, you can easily have gear that is too strong for low level content which is why a month after launch you see dps pulling entire rooms and surviving cuz they know heroic/normal dungeons are roflstomps when you are geared. Hell there's threads in the forums of people getting kicked because they werent ultra geared and were doing 1/5th of the other player dps because they were new while the older players had full on mythic gear and only doing heroic for legendaries/bonus or some quest.
In WoW gear is everything, people with gear have been selling leveling dungeon runs because they can literally pull half the dungeons and roflstomp it, very common in BfA's pirate dungeon where a single DH/warr pulled half the dungeon and killed everything in a few minutes to feed you xp while you afk near the area and then reset the dungeon, I know I bought a few leveling runs.
And in case you arent aware, the leave % in M+ is pretty high the moment a wipe happens and it means the timer will end, it doesnt matter that the run ends, people are often doing m+ for IO so if you cant time it, its bye bye for them and you can enjoy your depleted keystone.
And cuz i know WoW players like to attack others and pretend they dont know what they are talking about, feel free to check my profile https://raider.io/characters/us/frostmourne/Ralph%C3%A9