The issue is that when you have errors, it is impossible to determine if the error is at the caller, the callee or at the type signature. Essentially you have an equation of 3 dimensions with 3 unknowns. Same issue propagates into code navigation and refactoring.

It’s like regressing into a dynamically typed language except the errors are given during compile time. For small simple functions like the addOne example, it’s great to reduce boilerplate typing, but once you start relying on it for more complex types it can easily go out of hand.