Because in 90% of the cases the API isn't removed for fun but because something in the backend changed. You can't just "leave the existing" as that still means

Because in 90% of the cases the API isn't removed for fun but because something in the backend changed. You can't just "leave the existing" as that still means work if you have to write some translation layer from the old format to the new format. It's not like it would be free to keep the existing one around.