if i have an app built on Xamarin Native, and I have no time or resources to rewrite it on Forms or MAUI, will it generally work on imaginary iOS 17 and Android 14.
You will need to migrate your app to .Net for iOS and .Net for Android. You do not need to rewrite it to use Forms or the Maui UI.
See the Xamarin Support Policy. Italics are mine to highlight relevant sentences for your question.
.NET MAUI and Xamarin End of Support .NET 6 takes over from Xamarin for building Android, iOS, and cross-platform applications. Xamarin.Android and Xamarin.iOS projects should be upgraded to .NET SDK-style projects. Xamarin.Forms projects should be upgraded to .NET Multi-platform App UI (MAUI). Xamarin support will end on May 1, 2024 for all Xamarin SDKs. Android 13 and Xcode 14 SDKs (iOS and iPadOS 16, macOS 13) will be the final versions Xamarin will target.
MAUI is a broad term and includes both the Maui UI classes in Microsoft.Maui and the underlying cross-platform client stack such as .Net for iOS and .Net for Android. You can find the list of Maui SDKS in the .NET Multi-platform App UI (.NET MAUI) Support Policy