Yes it’s a hurdle, but it seems reasonable

99% of of users never get over that hurdle, which makes it unreasonable. "Monopoly" is the wrong term to use and it distracts from the issue - the better term is "Market Power". Google has enough power to have a potentially damaging impact on the industry. With that power comes responsibility to not do any damage - that's not just my opinion it's also the law (not in those exact words obviously).

Also - the apps are from "unverified sources" because Google deliberately refuses to verify them. They're happy to verify and assign a trust rating to every single webpage in the world... why are apps treated different? The simple answer is because Google makes more money by refusing to verify apps unless they share 30% of their revenue - which is basically extortion. There's no way they're doing enough work to justify a fee that high.

Sure, charge whatever fee you want but allow third party stores to compete fairly. In that world if they want to continue charging as much as they are now, they need to offer a hell of a lot more than developers are getting right now for their money.