There is considerable experience with doing large software projects in one shot. Many corporate and government software projects are done, or at least used to b

There is considerable experience with doing large software projects in one shot. Many corporate and government software projects are done, or at least used to be done, that way. Results were pretty bad. It's hard to be positive of causation, but correlation (long time to initial customer contact <=> software doesn't make customers happy) is pretty strong. If you want to challenge it, you could try to deliver a successful large multi-year software projects with no prior customer engagement, and document the process. People can then make informed comparisons against the accepted wisdom.