Before we all get giddy about the benefits of lactic acid one must understand some things. Lactic acid increases correspondingly to the acidic environment in the cell and although I agree that Lactic Acid is not the direct cause of force loss or fatigue the acidity that runs parrallel to lactic acid increase does.
So during endurance activty it's a friend but this relationship changes during heavy lifting and increases in lactic acid may not be so friendly.
As far as theories changing, hey it happens but in this case it's not that the theory changed, acidity still reduces force, they just pinned the tail on the wrong donkey.