Look, I'm not even going to argue Fields has been anything amazing. He's just been good, he's honestly just playing smart with the ball and making a few really nice passes here and there (a couple of which have been called back due to bad offensive penalties). But the things that went wrong with that offense in the 2nd half are completely irrelevant of who's playing QB.
In the second half, the Steelers had 7 drives. Of those 7 drives:
-6 drives had 1st down plays yield 3 or fewer yards (3 yard run, 0 yard run, 0 yard run, incomplete pass, 2 yard run and 2 yard run)
-2 1st down conversions were called back due to penalties (Harris run and Austin catch)
-4 3rd downs were from 3rd and 8 or longer, mostly due to the horrid running game
Now add on the other big plays Fields made that were called back due to penalties in the first half that were already mentioned, and it's very easy to see how Fields was getting no help from his team today. It doesn't matter who your QB is, most QBs are going to struggle with that kind of performance. Add on the awful WR group the Steelers have and it makes it even harder for QBs to convert in those situations.
Fields shares some blame there because he was very inaccurate in the 2nd half, but anyone coming to the conclusion of "a QB change would make things better" simply did not watch that game.