Thanks a lot!
I am part of the Amplication team; I'll try to answer your questions:
The first question is spot-on. We are already in the beta phase for .NET code generation, and it will be publicly available soon. You can contact us via the website for a personal demo of the .NET code generation.
For existing code bases - we do not currently READ any existing codebase. We can be used for scenarios where you want to modernize a legacy codebase into modern service architecture. One quick way to do so is with a DB-first approach, using our schema import feature (introspect an existing DB and quickly build new services with the same data models and APIs for them)