Which is not relevant. Monopoly is defined on a market, not on income segments (and even if it mattered, still on the high income individuals iOS is hardly above say 70%).
Someone could sell cars for $5K and have 99% of the car market in a small city of 1000 people, while another one sells a single car for 100M and has 20 times the other's revenue. It's still the cheap car guy that has the monopoly.
Revenue based market share is more relevant than unit market share for most businesses. Apple pulls in 2/3 of the revenue in the mobile app ecosystem and gets to act as a gatekeeper between most consumer startups and their potential customers.
Someone could sell cars for $5K and have 99% of the car market in a small city of 1000 people, while another one sells a single car for 100M and has 20 times the other's revenue. It's still the cheap car guy that has the monopoly.