You mean like Phil Mickelson, John Daley, Bubba Watson, or Tom Watson in his prime? Are you going to tell one of those guys their backswing is too long and is a problem? How about me? I have a long backswing. Way past parallel. I also hit 83% of my fairways and the regularity with which I split fairways gets comments all the time.
It's not the length of the backswing that is the problem. Doesn't almost every golf instructor say to turn until you can't while maintaining your form? But there are faults that produce a long backswing. Collapsed arms. A poor grip that doesn't support the club at the top. A reverse pivot. Manipulating the clubhead instead of allowing the club to swing. Snatching the club inside resulting in the club going way across the line. Tempo issues. These are a few swing faults that can present themselves as a long backswing.
We have to separate symptoms from the cause.
Short backswings aren't necessarily a fault either. Jon Rahm would be the current poster child for a short backswing and he does pretty well with it. Most professionals with a short backswing get distance through steep shoulder turns, down cocking, and really aggressive acceleration on the downswing. Jon Rahm is actually pretty unique in that he has an incredibly bowed wrist at the top and at impact. His swing is also way across the line at the top, but many don't notice this because he is so short of parallel.
It's one of the most pronounced problems with my swing right now. There's no reason I should be swinging with that range of motion on the course since it just ends up with not being able to engage my lower body for rotation in a reasonable amount of time.
@Canadan: This is probably going to seem presumptuous coming from a high-handicapper and brand new golfer that hasn't even swung a club yet this season. FWIW: I've spent a goodly amount of time studying and discussing swing mechanics, and I've been taking off-season training from True Motion Golf. For what all that may be worth, this is what I see:
Your lead arm is breaking very badly on your backswing. Your lead arm breaking so far is also why the upper part of your trailing arm is rising so high. You're very disconnected at this point. This may help cure that: Tac Tic Elbow Golf Swing Tempo Trainer. Whenever your elbow breaks too far you'll hear and feel a pronounced *click*. (The thing looks uncomfortable. It's not.)
That device also aids follow-through, because it'll alert you to the fact you're breaking your lead arm too early in follow-through.
Going further out on the limb... Also, ISTM that, unless you're initiating your downswing with your arms, you must be losing swing speed because, otherwise, when your hips get to where they need to be at P7, you'll stall while your arms catch up?