Competition might be better than you think, Walmart captures less than 10% of U.S. retail, Amazon less than 7%, and Target is behind Amazon. But if you find the notion of voluntary exchange for mutual benefit immoral, it hardly matters where we shop. Thanks for sharing your thoughts, I enjoyed hearing your perspective and don't want to provoke any shouting :)
It's hard to define and measure power, but let's set that aside. Many people don't have enough money to buy a house. The usual solution is rent. Probably we will agree that someone who owns houses has more power than someone who doesn't have enough money to buy one house. So the landlord has more power. Under your rule, a lease will be invalid because of the power imbalance. If the landlord knows that the lease will be considered invalid, they have little reason to let the tenant move in. The lease gives the tenant more power, to assure the landlord of a credible intent to pay, compared to a tenant who cannot make a credible promise in the form of a lease. How are low-power people supposed to rent homes, buy cars, or get jobs if the high-power people know that promises they make will not be considered binding?