I don’t understand why this is an issue. The “lock-in” is only perceived by the theme user, it’s not real, especially when the content is owned. Any theme could add a very basic option and method to convert post type names. The answer should be to build better tools to convert their prospective customers; to help those that might think they’re locked in. It’s good business for Array and others to help convert than to wait and expect others to follow 1.
The real lock-in isn’t custom post type slug names: it’s post_meta 2!
We shouldn’t give up control by imposing any standards on a popular plugin. Jetpack shouldn’t be an authority for standards; as well as s2Member should never be an authority on subscription logic (even if it was extremely popular).
As I wrote in my response CPT slug names really isn’t an issue. If these businesses want to help customers (by converting them into buyers) they need to provide tools to help them convert from older themes. Technically a CPT slug name is probably the easiest thing to change and an interface to migrate is a days worth of work–more importantly it’s a business opportunity.