Even with the saddle sitting balanced, the more modern, deep seat saddle has an excessively curved tree. With the correct width, rail angle & balance, this saddle will not cause any muscle wastage.
However - the shape is more rigid and curved towards the horses back - which wouldn’t naturally encourage a horse to lift it’s back into, possibly making this part of it’s schooling take longer. You would find that wool flocking will crush (especially in the middle of the panels) requiring constant repacking on a regular basis.