Because every feature (including optimizations) starts at minus 100 points . This is especially true if the semantics are different, which they are in this case .

The difference is subtle, but if you define Foo as:

Task Foo(string text)
  throw new NotImplementedException();
then the behavior of Proxy and ProxyAsync are different. Proxy would propagate the exception directly to the caller; ProxyAsync would capture that exception and place it on the Task returned from ProxyAsync.
I have a blog post that goes into more detail on thoughts to consider when eliding async and await.
                                        
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