Agreed. Mixing business logic and UI works in simple applications, prototypes, or applications intended to ever work with one platform and UI paradigm. Outside of that, you're supposed to keep your "business logic" and UI separate (isn't this exactly why people invented MVC?).

Maintaining "5 different native UI applications" should involve one codebase with the whole internal logic, and 5 separate UI layers mediating between that logic and the platform it's being run on (including UI). It should not involve rewriting the same thing 5 times.