As a 3rd-party developer I could almost understand retiring apps (too many dependencies on deprecated stuff, ancient code base is too hard to maintain, etc.).

As a 3rd-party developer I could almost understand retiring apps (too many dependencies on deprecated stuff, ancient code base is too hard to maintain, etc.). Yet built-in apps are much more special. For one, many people consider built-in apps to be "the OS" as much as the OS itself. And two, if there is any development team that can continue supporting an ancient code base, it's the OS vendor: if they really have to, they can do things other companies can't (like privately continue to ship functions that are now publicly unavailable). It should always be more in their interest to evolve rather than redo.