So why don't you just define a non async method (A) and then one that is async (B) that calls A? If you want to run async then call B if not then call A.

If you could define a method as async and then call it either syncronously or not the whole point of the async keyword is lost. You would end up with the compiler doing a load of work because every developer will just slap async on every method just in case rather than actually thinking about what needs to be async and what doesn't.

Adding the async keyword is not a magic bullet, there's a huge amount of work going on under the hood every time you use it and it's important that developers appreciate that.