Depending on the implementation, WCF can be really easy to adapt to new clients. If you wanted to support Linux, macOS, or web, you just implement the part of your service that make sense for those platforms.

I obviously don't know your app at all, but it sounds like a 10 person dev team could probably build a new app in just a few months since the backend is already there. It wouldn't have all of the features, but generally speaking it's a lot easier to rebuild an app than refactor an existing one. Whether that would bring value is another concern entirely.