I also doubt that at this point any plugin that sets out to prevent a customer of the lock-in effect will gather enough momentum, to be of any significance and real help when it comes to switching themes. With hundreds of independent theme shops, it is extremely unlikely, that a customer switches to a new theme, that is build on this exact same plugin. Therefor WordPress is probably already way too widespread.

How about a plugin that can export my otherwise locked-in data, and that has the functionality to create the CTPs, taxonomies etc. that I want to use again in my new theme?