I love Xamarin, which a cross platforms mobile development framework. I can use C# to build apps for iOS, Android, and Windows. However, it doesn't mean I can get away from learning Android and iOS development. Using Xamarin is not always the best option. If we know the native platform, that should be more flexible. For example, an app only planned in running for a single platform, that may be a better choice to be built in native or if the app is too complex or memory critical, I suggest that should be constructed in native. That is a better way to utilize the resources.