Native iOS and Android developer here. I switched to Xamarin a few years ago. As a native developer i was afraid of Xamarin and that i couldn't harness the power that Swift and Java gave me. However it give's you full control over your apps just as Java or Swift does. Everything you do in Java or Swift can be done with Xamarin in C#! Your knowledge isn't lost, the syntax has only changed. Nowadays i absolutely love it! I work with enterprise companies as a consultant using Xamarin, code sharing is approximately around 80% when using a Xamarin and a framework like MvvmCross! In my opinion Xamarin is the feature of building apps. Please ask any questions you have and i be happy to answer.