When parts of a market become dominated by one or few companies operating in a limited choice environment, consumers can't just opt to not use both Apple and Play store. You need to choose one in practice.

At this point the regulators should investigate what the barriers are to new entrants and if it's too costly and nobody has managed to cut in the last few years, establishing some rules is probably a good thing. This happens as industries mature and become critical, it happened in transportation (most bus, train companies), energy, water supply, trash, etc, depending on the country and market conditions.