Arm Structure Beyond Impact
I would love to say that all you need to do is aim for the 'L' shape but this is a very complex action that is affected by all the movement that precedes it. If you are too wide in the downswing (early right arm extension and ulnar deviation) then you will be pulling your arms in through the ball. If your brain detects that the club is coming in closed then again, you will have a different arm structure post impact. So when viewing your swing and observing sloppy arms in the follow through you must take into consideration what went before.
In some cases, by working on the follow through in isolation you may improve the pre-impact movement too but this will only happen with a lot of messy strikes and inaccuracy on the range first. I would take this approach with a lot of golfers while some may need to work on the pre-impact movement before this can clean up. I would take this on a case by case basis.
Some don't re-hinge (link to a new video)
One of the most predictable themes that I have noticed over the years, is that high handicap golfers focus their attention mainly on backswing and impact with very little awareness of what occurs, or what they intend in to happen past the ball. Elite level players have very clear intentions and awareness past impact. Whenever I see a strong groupwise theme such as this (albeit observational) I can't help but think it is significant to performance.