So, the answer to the overall question is that physics requires an apportioning of bandwidth between up and down, but economics can here route around physics: If a provider were committed to providing symmetry without lowering download speeds, it could have (for example) put in a second line when it put in the first. The trade-offs were made by the access providers, largely based on business concerns. For example, one reason we have asymmetric fiber may be that the providers have been charging such high premiums for symmetric access that they don’t want to lose that extra money, even as the technology makes symmetry more plausible.
