In theory you could hope that you find users and afterwards offer consulting and support services for your users. As well as charge for development of custom features/extensions that they need.

I guess in practice I think business models around libraries, frameworks and infrastructure software are hard. There is often lots of competition. And users in the meantime often expect that these things (including support) are for free.

There is none. We develop ember because we need a tool to solve our problems and the existing solutions weren't good enough in at least some aspect that mattered.

The idea is that opensourcing software makes people you don't know can use it, improve it and ultimately you get those improvements yourself.

Well there's a group of people in the Core Team. I know at least one of them has been hired by their company (LinkedIn) to work on the framework full time, as their app (and subsequent business) is built on top of it, so it makes sense to have someone paid to focus on it.