It took Apple 12 years to deprecate Carbon API. It is still being shipped for compatibility.
Nobody really throws away anything, that other people are depending on. Especially, if you are depending on their continued investment into your platform.
Otherwise, Windows Mobile to Windows Phone happens. Or Python2 to Python3 (how many years it is already taking?)
So two major releases is too optimistic. Especially in android world, where more than two major releases are used simultaneously.