I would point to Craig Kimbrel throwing from his knees after his ankle broke and how it benefited him. And it benefited me when I had my bad knee and after my knee scope.
Before my knee scope my max long toss right around 300 ft and after my knee scope and throwing solely off my knees for a solid month(maybe 2 I don't remember quite how long) my arm action and upper body rotation improved to the point where I'm able to get out to 320-325 ft. And hopefully further once I build my arm back up from having a big ole' not above my collarbone.
So I wouldn't knock throwing from one's knees. Constraint training is a valid and effect method of training.
And actually most velocity comes from the upper body's rotation. The fact that I personally long tossed from my knees at around 200ft shows this, and I believe people like Tom House say that 80% of velocity comes from the upper body. But don't quote me on that.
Just some food for thought.