My brain is tired and I'll say I'm an average golfer and me focusing on my left arm is like anything you can think of that is feels wrong. If I focus on right arm, oooh the natural righty takes over, and cast happens and rely on perfect timing.
So, for me, I focus on my grip and keeping it oh so soft, esp right hand. Get a good take away, stay soft, and focus on my shift/turn and no early ext. The rest happens...just like my soft right hand and arm will get tense when it needs to.
I'm not good, just an 18, but I know I'm an 18 because my short game is awful, and I'm working on it. Not because my right or left arm does this or that...
I'm never going to have a perfect swing, but I know I can get to where I can trust it and it be repeatable.
Golf is hard already, why do we make it harder by focusing on so much?