I just think Mayfield is incredibly limited in what he's able to do when he's under duress.
If Beckham gets open and Mayfield drops his eyes before he can make that read and ends up scrambling into a sack, that's on Baker.
If Baker reads the coverage presnap and decides before the ball is ever snapped he's going to throwing to someone else without letting the routes develop to see if someone else is going to be more open further down the field, that's on Baker.
You can only do so many gimmicks like play action boots and moving pockets and even when you do those the ability to get YAC is usually pretty limited.
At some point Baker just has to be able to navigate a pocket and go through progressions.
Here's an example (this will become much more clear when they release the All-22) of where Baker really struggles.
Wills gets beat immediately with a speed rush to the outside. Not Baker's fault, but he must step up in the pocket or he's getting sacked.
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For once, Baker actually DOES step up into the pocket and away from the pressure, but ultimately it's meaningless because he steps up with his eyes down desperately just looking to scramble forward. Because he's is so afraid of getting hit, he completely misses Landry who made a beautiful route adjustment and is literally running wide open directly in front of Baker, just 7 yards ahead.
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If Baker doesn't completely panic under pressure, this is a throw he can and should easily make to Landry in stride for a first down. Yes he's going to get hit, but he got hit anyway on the sack so what's the difference?
Since he immediately dropped his eyes the moment he felt Wills get beat on the blindside, he never even saw Landry and just skittishly scrambled into the sack.
Mayfield just has not shown that he's really capable of making many positives things happen from inside the pocket when he's under duress. He's good in scramble drill situations when he can break the pocket because that gives him a much more clear window to throw and he does throw accurately on the move, but he's also not the elite athlete that Russell Wilson and Kyler Murray are, so he's not able to get into those scramble drill scenarios as easily as they are.