And it turns out that much - often most - of the value in software isn't in what we planned, but what we LEARNED from what we planned. So we need to LEARN as early and as often as we can. That's where the gold we're panning for usually lies.
The last 30 years I've been doing software development has been characterized by those user feedback loops getting shorter and shorter. In the mid-90s, my teams would tackle a bunch of usage scenarios, getting feedback every couple of weeks.