It's not assuming the market is competitive. Wireless markets are often not competitive, because entry is limited by scarce spectrum availability etc. which incumbents lock up to limit competition. The point is that if you had a competitive market, margins would be thin and the only place for lower prices to come from without a reduction in operating costs is higher prices somewhere else.
It's easy for regulations to lower costs in an uncompetitive market because the lack of competitive pressure allows the incumbents to do all kinds of inefficient nonsense that would drive them out of business if it was actually practical for new competitors who don't do that to enter the market and undercut them on price. But what you want to do in those cases to the fullest extent possible is to restore competition to the market, not try to regulate the incumbents while keeping them as a monopoly/oligopoly. A set of regulations that gets you 8% of the benefit of actual competition can be a significant improvement from the status quo while still being by far the less effective solution.
It's easy for regulations to lower costs in an uncompetitive market because the lack of competitive pressure allows the incumbents to do all kinds of inefficient nonsense that would drive them out of business if it was actually practical for new competitors who don't do that to enter the market and undercut them on price. But what you want to do in those cases to the fullest extent possible is to restore competition to the market, not try to regulate the incumbents while keeping them as a monopoly/oligopoly. A set of regulations that gets you 8% of the benefit of actual competition can be a significant improvement from the status quo while still being by far the less effective solution.