how easy is it to migrate a BaaS approach like this (stamplay, firebase, parse etc) to a self hosted one once things start to scale beyond 10,000 or so users?
It depends from the service, everyone has its own lock-in. With us you own the data and you can take it anytime, the hard part is to code the backend from scratch on your own.
Thanks good to know. Will give this a try for prototype and then roll our own backend later.