Holding down the shoot button decides power only, where you aim is where the shot goes, except it then takes into account your position, how much pressure you're under, which foot you're shooting with, your finishing and shooting stats and the conditions the match is played in (ie rain/snow would make things a bit treacherous) to actually decide the outcome of the shot.
I personally disagree a bit with this solution. I agree that power alone should be on the Shot button. The problem for me is that your solution seems to take away the ability to decide whether you want a high shot or low shot.
The factors you mention, position, body balance, etc, all help to decide the outcoming shot, but the biggest factor for the height of the shot is the way the player hits the ball and shapes his foot. This aspect, as in real life, would be nice to have controlled by the human player.
So, my ideal system, and a system I feel they should implement as a possible way to play the game (if they can cater to 2-button novice players, why not to to the other extreme):
Shot Button: Power
Stick tilt: Height
I'll add that this system was used in ISS64/98/2000 for the N64, and it worked like a treat, and it is surprisingly intuitive once you get the hang of it.
If one were to make the argument that the tilt control would be difficult to use in crowded situations, or situations where you barely have enough time to hit the Shot button, much less fiddle with the stick tilt angle, they could make this whole function controlled by the LT trigger.
So
Normal Shot:
Shot button: Power & Height (filling bar more gives more power and more height)
Stick direction: Aim
"Advanced" Aiming mode: Holding LT at any point between starting to power up the shot, and the shot being executed (or before shooting, obviously), will allow using stick tilt angle to fine tune the desired height. Player stats and body balance/etc/etc will make this desired height more or less achievable.
Certain shot situations, such as right after the ball bounces, will make low shots impossible despite tilting stick very little.
The above was actually exactly what was used in the N64 ISS series. There you held The Right Bumper Key to enable Analog height control.