Creating an async application or sync application is an architectural decision. And since architecture is the sum of all decisions that are hard to change (I think this is from Martin Fowler) thus decision - async or sync - is hard to change and one must live with it.

For applications that is true but for libraries it often is not. You can e.g. design REST API libraries quite well with most of the code ignoring sync vs. async completely and only the part that actually performs requests in a general way has to handle the distinction which is why many of them include both versions.