But 90% of the users (me included) use only the play store and barely know anything about the rest, let alone trust them (me included again).

"People don't use them" is qualitatively different from "they cannot exist", though.

I'm guessing you don't have a source for that because I would be shocked if it were any less than 99%. It would be wonderful to see genuine rivals, offering alternative business models.

The only significant one would be Amazon's app store, which doesn't really offer any improvements over Google's.

I use f-droid over te play store whenever I want to install an app.

Yes, there are others, but I imagine (I would like to be wrong) that Amazon is the only one with any significant marketshare. Maybe something in China.