> It’s an irrational proof of commitment.

I think that's a bridge too far.

There's many clear reasons why people want a diamond, and even an angle like "I want to follow society's expectations" is to me far from being irrational when you're prepping yourself to build a whole life with someone else.

A couple can discuss these matters and come up with alternative paths, people are referencing other materials and I'd include not buying anything fancy and save up for the first payment of their home for instance. But brides requesting a diamond is usually far from lacking rationale.