At the level of customer service they provide (ie ~none), Google Play would be "sustainable" at a 1% cut. Everything above that is pure profit. Which is exactly why they're doing everything they can to lock people into Google Play and lock people out of distributing/accessing apps outside of it. This isn't some kind service Google is providing at cost out of the goodness of their hearts, it's their attempt to corner the market and extract a massive amount of economic value without doing any meaningful work.