I hate to break it to you but "fully defined requirements analysis" is a pipe dream filled with rainbows and unicorns. I have never, not once, seen a requirements document that accurately captures exactly what the system will do.
Well, I've written them, and I've never had a project fail.
One example involved interviewing
i) the owners of the company
ii) an executive from each department
iii) a "regular joe" representative from each department
This became a 40+ page project specification, which was signed off by all stakeholders and became the contract.
Then this document was fed into a series of code generation engines, which created hundreds of thousands of lines of code. This was all done with an eye towards allowing various professionals to go away and do what they do best without getting held up waiting on each other or tripping over each other, filling in the missing functionality in the generated code.
That system is still in operation close to a decade later, organizing the working lives of thousands and serving the needs of millions.
Now I work in Agile. I hate it. I'm always having to check with other people constantly to move forward, I never get in the zone, there's a lack of clarity and vision, and I feel like I'm getting stupider each day and I'm not producing my best work.