That is in fact exactly the trap door of open source -- you are QUITE WELCOME to switch to a non-opensource model once you've reaped the benefits, but guess what, anyone else is also QUITE WELCOME to make a still-open-source fork at the point you do that.

This isn't an accident, it's in fact the whole point of open source that people can make open source forks when the original authors decide to no longer do open source.

I don't think there's much 'amicable' about it though in this case at the moment.