The current generated state-machine mechanism is not going away any time soon. Instead, Roslyn as well as any other compiler that targets CIL is expected, when appropriate, to stop generating state machines and start generating specially annotated method calls recognized by .NET.

This means that all existing libraries will continue working as is, and newly written or existing code that is compiled against e.g. C# 14/.NET 10(?) can automatically opt into new mechanism without user intervention. The write-up does cover the considerations for interoperability between these two models as well.

In fact, this will make using async from other CIL-based languages much easier as the burden of capturing the state and suspending/resuming the execution will be on the runtime rather than language compiler authors.