You said "Seven percent is generally accepted as the expected market return over a period of time sufficiently long enough to mitigate systemic risk, ". Also, When people say "historical return is 7%", they do not say it as a historical fact, but as a forward expectation like you did.
So, you need to provide why 7% is generally accepted, and why not 2%, or 70%.
Comparison to gdp and inflation definitely apply. Business profits in an index grow with Inflation and gdp growth, and stock value is the expectation of future profits discounted over time.