It's happened to me in real life. When you want to go from keeping a value in external storage to keeping it in memory, you've gone from async to sync. The reverse happens too. These are not huge conceptual changes. When they require restructuring an entire program, it's reasonable to wonder why that program's structure is so brittle. "Likely make them async even if you don't know, just in case" sounds to me like an admission of this problem. Why would I want to make an in-memory lookup async, thus forcing it to wait in the event queue and defeating the purpose of RAM? The only reason to do that is that the programming model imposes a large complexity tax for not doing it.
Consider the simple case where one wants to look up a value synchronously if one has it in memory, and go get it from storage asynchronously if one doesn't. That's a natural thing to want, but it's problematic in Node. The problem is not syntactic—you can easily write a function that calls back immediately in the one case and asynchronously in the other. It's that sync and async semantics don't compose well, so when you start to do anything a little complicated (e.g. launch N requests and call back when all N have either returned or failed), the two trip over one another. Working in a Lisp that compiles to JS, I had to write some surprisingly complex macros in order to get correct behaviour. I wouldn't dream of writing that JS code by hand.