"What if we're building a bridge," did I hear you say?
1. You're probably not
2. You never know what you'll find until you start digging
3. Even bridges come with stress sensors and smart motorway signs these days
Unable to easily cut production code, Royce advocated 'simulation' to reduce risk: "... the pilot effort could be compressed to three months perhaps... [the personnel involved] must have an intuitive feel for analysis, coding, and program design. They must quickly sense the trouble spots in the design, model them, model their alternatives... and finally arrive at an error-free program."
Agile understands that we always learn by doing. Either unintentionally, clumsily and expensively. Or intentionally, with control, and efficiency.
The choice is yours