[BjoernKW]

I draw the line where it makes sense from a business value perspective. If the product you're working on is a mobile application then it makes sense to both know mobile and back-end development.

Personally, I work on business applications exclusively. Creating value in that realm nowadays usually involves a web front-end and a back-end. The front-end in these cases often is required to run on mobile devices, too, which however is perfectly achievable by using web-based techniques.

Occasionally, there's the need for having a dedicated mobile app. In those cases, I make use of web-based frameworks such as Ionic. Not only do those provide the additional benefit of staying platform-independent but they also abstract over the specifics of mobile platforms.

This might not be the perfect approach in each and every case but for business applications it's often more than good enough and the little additional value created by maintaining a dedicated Android or iOS codebase doesn't warrant the additional effort required to do so.