In reply to Dr Gallagher.

Because a CPT can literally be any type of content/data, there could not truly be a guarantee of that, at least in terms of how the average end-user would see it. For example, if nav menus were a plugin instead of baked into core, you'd need massive theme adoption for them to work because the nav menu taxonomy and nav menu item post type are completely different from your typical posts/pages.

If the CPT worked basically like a post or page, switching themes should typically be no problem.

You'll never technically lose your CPT content when switching themes. The conversation is just a bit more nuanced than that though.