Natural development plus the addition of Marshall makes it a logical conclusion that Geno would be a better quarterback in year 3 with Marshall than he was in year 2, possibly a lot better. And he would have needed to be a lot better to play at the level Fitzpatrick has played.
The problem with Geno was always the mental mistakes. While Fitzpatrick doesn't always play that smart on the field, he plays smarter than Geno does, and he isn't prone to making game-changing mistakes like Geno did. Fitz has done a fantastic job avoiding sacks this year as well, which has helped. And he's done well in the red zone in situational football.
I think Geno would have moved the ball just as well between the 20's as Fitzpatrick. But there are too many dumb plays that negate anything good he might do. We saw all of that in the Raiders game - he moved the ball pretty well, but he made some really dumb mistakes.
I also don't want to think of what our offense would look like without Marshall, so I refuse to think about how Fitz would play without him.
Goddamnit, I didn't want to respond to this debate and here I am.