Not only does this approach help you determine what the backend actually needs to do/acquire, it has the added bonus of getting product teams to iron out critic

Not only does this approach help you determine what the backend actually needs to do/acquire, it has the added bonus of getting product teams to iron out critical business logic (which masquerades itself as the frontend) while you transition to backend work. This is a huge timesaver, because if product decides to change the frontend, you haven't committed huge resources to the backend. Building software front-to-back helps people agree on what needs to get built.