You really have no idea what you're talking about.
Rescaling every single ability in the game based on the level of the dungeon is something you do during the development stages of your game. I can't think of a single solitary game that added a system like this later that wasn't a freakishly old game like FFXI or a brand new game like GW2 (meaning at launch, I am not saying GW2 is a "brand new game"), which both use drastically different systems from FFXIV for their combat.
It's more than "simply unlocking the skills," as many of those skills use percentages, which would have to be either capped or readjusted. And you'd have to do this for every single skill in the game. And then you'd have to test it.
All for something that would absolutely be exploited by people selling runs. There were tanks whoring themselves out when NIN came out to decrease queue times for dungeons for DPS classes. Not to mention people selling coil runs, which is a major motivation for designing several of the FCOB fights and the loot system ("preventing carries") Do you honestly believe people wouldn't sell runs through dungeons?
IF this was something in the game from the beginning, that would be fine. But that's NOT the decisions they went with, and backtracking would take such an absurd amount of resources that could be spent elsewhere that if anyone should feel embarrassed, it's you for defending the idea so vehemently in the first place.
What you're doing is akin to looking at the Washington Monument and saying it should have two domes adjacent to it to recognize the great accomplishments of John Adams. He already has a monument elsewhere (sorta) and adding it onto the washington monument would make it look silly.
And you want it to work differently for Trials than everything else.
That's even more coding and testing.
And before you say "it's easy," it really isn't.