Truthfully, I never thought he would be as horrible as he has been in a backup role. Granted, his role changed four plays into the season, which is so Jets. But still...
As someone who has played the game, and coached the game, I will say there are certain things that I look for first and foremost in a QB and unfortunately it is extremely difficult to train (most who can't do it, never can). The way I always evaluate is
1. Ability to read a defense
2. Ability to anticipate a throw
3. Arm strength
4. Athleticism
It is literally in that order. If you can't do #1, I won't both to evaluate 2 --> 4. #1 & #2 are the most important and what makes a player an actual QB. Unfortunately Zach does not have these skills and never has, and has shown no ability to develop in that realm.
This is not something that has improved over the past 3 years, it's the same as it was when he was a rookie. Off script works for him because he can run around and look for the open player, but he simply has issues reading defenses, looking off defenders, and making any timing and rhythms throws.
I didn't see these traits in his highlight reels from college and didn't like the pick from day 1. That being said, here we are three seasons later and Zach still looks like the same player that came out of college three years ago.