but would 20 hours additional dual, be less safe than 20 hours of solo? Its a question of the safety benefit for the economic argument. It could even just be, good old "common sense"
You only have to take a look at the ratio of dual to solo hours 40 years ago. Then it was normal to see a pilot's log book showing 50 total flying hours which included 30 solo. Now you will see a typical spread of 50 total flying hours which include maybe 10 solo. It is solo hours that are most important because that means decision making time. Once a pilot completes his first solo it used to be normal that each following dual hour would be followed by one solo hour as the student consolidated what he had been taught in the previous dual session.
I have frequently seen student log books where after first solo, the next 5-8 hours are dual followed by another short solo and so the ratio of dual to solo steadily seems to increase in favour of dual. Often this is due to some instructors "milking" the hapless student to pick up more money and/or to fill his own log book. CFI's will either know what is going on but keep quiet because it is money going into flying school coffers. Or they fail to closely supervise the instructors while the student gets stitched up with more dual. Usually the latter.