I've learned that in Oscar's world, when he says not to focus on the feet, he doesn't mean that it's not important. Rather, the cues he provides will lead to the feet and body doing the right thing.
For example, I struggled with volley footwork for a long time. I finally tried Oscar's cues, which take the focus back to the hand, and his cue that one leads with the head - yes, I mean the head with two eyes in it, not the racquet head.
When I did this, presto, the feet did the right thing, and now I just don't think about it. The physical reason I think is that when one leads with the head, the body goes out of balance, and one is forced to take a step - and there you have it, the volley footwork!
Similarly for the forehand, being in open stance forces one to line up with the ball using the outside foot.
All this said, I still believe for tennis beyond the club level - where most of us will probably never go - coaching and fine tuning of all aspects, including footwork, is essential. But it's probably going too far to treat footwork like a dance that needs to be choreographed.