Sigh, this was the basis of "Iterative Development" that came along with the "Rational Unified Process" and was the precursor to the current fad of epics/storie

Sigh, this was the basis of "Iterative Development" that came along with the "Rational Unified Process" and was the precursor to the current fad of epics/stories/sprints/t-shirts/Fibonacci that has polluted current programming. People won't follow it because it means reviewing their assumptions and re-planning, but there are always external entities that will not or can not do so (other departments, finance, customers, marketing, sales, etc etc).