It sounds nice, but in reality your requirements end up being a castle in the sky. My team once spent 6 weeks for a small project (I was the team lead) creating

It sounds nice, but in reality your requirements end up being a castle in the sky. My team once spent 6 weeks for a small project (I was the team lead) creating a detailed requirements document with screenshots, a prototype app, etc. My users were all at hand and I showed them the prototype. They all agreed that it was what they wanted, and signed the requirements doc. We spent several months building the application. When we shipped, the software was unusable, because a security feature that we had been explicitly told could wait for the second release turned out to be essential. We also ran into a big problem with concurrent editing that no one had considered in advance, and blew up our carefully-created design. The solution to changing requirements isn’t to nail them down, because that’s effectively impossible, but to get better at evolutionary design techniques (introduced by Extreme Programming) so you can adapt to arbitrary requirements changes without disruption.