"Why is it worth the switching cost?" is the genuine question I have too. Or more importantly, why not just make it ProseMirror v2? The landing page needs more information about the "why" rather than the "what".

From the blog post:

> A ProseMirror 2.0 with an incompatible interface would amount to the same but make it ambiguous what people mean when referring to ProseMirror. Trying to graft stuff on in a backwards-compatible way as an 1.x version would produce a compromised win32-style mess.