I’ve worked with plenty of valuable apps not using async. If the requirements change such that async becomes the best way to solve the problem, that’s something the business would decide at that point.
Sometimes you can think ahead and see the need coming -- or not coming -- and act accordingly.
Experience is helpful here.
When I have to write C I try to at least write thread-safe C close to something I could expose as a library. I've had too many cases where I or someone else wrote code they NEVER thought would be needed as a library, therefore took all sorts of liberties, and then suddenly there was a need for a library to do what said code was doing. Oops. I've run into fewer instances of sync code that turns out should really have been async and where adding threads isn't good enough, but I've still run into some.
This isn't about functionality -- this is about scalability. When you know you need to scale, you have to think ahead about it.
A lot of projects are little more than crud with a few complicated business rules. Use a framework with a good caching layer, and a lot of the time the employer is very happy with the outcome. It’s cheap to build, maintain, and host.
But then along comes a developer who says “we must build this app with async so we can scale”. Suddenly the junior devs build an absolute mess because they don’t know how to work effectively with async. So a few seniors get involved. Then the project runs out of cash.
Sure, some projects benefit from async and some fundamentally require it. But not all of them, I doubt even the vast majority of them.
You only need to scale when you’re successful, and most applications are not successful.
I’ve worked with plenty of valuable apps not using async. If the requirements change such that async becomes the best way to solve the problem, that’s something the business would decide at that point.