When developing web apps, it is not possible to write the code for the alternative platforms aswell. We are not just talking about changing tiles but also alot of manipulations with the maps in terms of layers, polygons, routes, markers etc on the map.

What you are saying is already done in places where it can be, for eg, use of ORMs allow the application to be connected with a variety of DBs and your DB queries and application become independent of the underlying DB.