Hi, the author is trying to prove that solar is cheaper than traditional. Their rationale is that solar has gotten ten times cheaper, and traditional has become more expensive.
This does not prove that solar is now cheaper than traditional. If solar cost 100 dollars per unit 20 years ago, and traditional cost 1 dollar, then the present prices would be 10 for solar, and somewhere greater than 1 for traditional.
In order to fix the argument, we would need to prove that solar cost less than 10 times traditional 20 years ago. If this were true, then solar would be guaranteed to be cheaper now.
