Not true from what I've seen Every C# library with support for async/await has synchronous versions of those same methods. In fact, Microsoft's own documentation recommends appending "Async" to the method name for precisely that reason - to distinguish an async method from a sync method.
Furthermore, "asynchronous" code doesn't need separate threads. A scheduler and green threads is more than enough.
Have you seen Go? You should take a look at it. It makes writing concurrent code basically trivial, using "green" threads and channels and a neat little "select" statement.
They only support both because most libraries still support older version of .NET
If they were to support only .NET 4.5 or higher, then you can just call any async method in a sync manner simply by doing httpClient.GetAsync(...).Result
Furthermore, "asynchronous" code doesn't need separate threads. A scheduler and green threads is more than enough.
Have you seen Go? You should take a look at it. It makes writing concurrent code basically trivial, using "green" threads and channels and a neat little "select" statement.