They're really not notable alternatives. I love F-Droid but its selection is very limited. Good luck finding people who use Amazon Appstore, Huawei AppGallery or any of these other stores.

Let's imagine there's a company called Ticketmuster that had a monopoly on ticket sales. If they tolerate a shitty little kiosk selling a score of tickets a day, does this mean Ticketmuster does not have a monopoly share of the market?