Option 1: You scale the Lv90 to perform on <example-dungeon> at for example Lv50 to perform as well as the Lv50 player fresh in the instance. You now have the same level of performance pressing way more buttons but no additional output, giving you essentially a disadvantage by having to work harder. Not terribly hard to deal with, but would still be meh.

Option 2: You don't scale as closely and the Lv90 player has a numerical advantage over the LvXX player. The LvXX player will feel probably bummed out for being so weak compared to his high level peers and feel the other player may have an unfair advantage if they feel even remotely competitive.

Additional note: Extremes and Savages of old are still content being done, even if sparse. Furthermore, the amount of tuning work needed to be done to scale kits of high levels to do roughly the same performance of that of low levels is a balancing act way more complicated and dynamic than having a static range of abilities for each dungeon / trial.

We are getting Unreal Trials for such reasons, what we could ask for is Unreal Dungeons perchance. Then again probably better to spend the resources on the upcoming Variant / Criterion Dungeons.

But that is just my two cents.