Sheltron was a bigger difference maker in A3S than it is in any of Midas. Sheltron was also better in Ravana EX (which was the hardest fight in the game before Gordias was released), Thordan EX (which was the third hardest fight in the game when released), and Sephirot EX (which you could consider not end-game content, but I think if a fight is one of the 5 hardest fights in the current content cycle, it should count) than it is in Midas.

There are perfectly fine ways to showcase the merit of Sheltron. Midas is not that way. If anything SE probably purposefully made Sheltron worse in Midas as to help balance PLD and DRK's viability in content. They probably saw how much DA DM threw a wrench in PLD vs. DRK mitigation and didn't want to repeat that error except in favor of PLD.