I take your point about usability as opposed to visual design, and that many slick-looking UIs actually lower productivity.

But:

In this example, unless not handling international phone numbers leads to failure of the project, that can be handled later, say once the project is approved and time estimation is being done. If I'm building a notes app, and someone is proposing a new sign up form to increase conversions, and it has a phone number field, handling international numbers is the last thing to worry about at this stage (unless international numbers are a significant problem with the old form leading to abandonment).

We shouldn't doom good ideas with irrelevant details, which are absolutely relevant later, but not now. Product development happens in phases of increasing fidelity, and issues need to be brought up at the appropriate time, not too early, not too late.