I would claim that if something requires 8 man years, that it will most definitely take more than a year to develop with 8 people.
Communication takes time, coordination takes time, there is an incremental cost to each news person added to a team. From experience, perhaps with 2-3 people who happen to gel well together you may get close to proportional scaling of output, but with 8 it’s really unlikely in the real world.
On the contrary, such linear scaling would be quite exceptional. I'm speaking from experience but you don't need to trust me; I invite you read any book on software engineering management, starting from The Mythical Man-Month by Brooks.
the mythical man month was first published in 1975, I think the typical applications programmers work on today have changed significantly since then and encompass many different disciplines (to be thought professional) - so many disciplines that one developer is likely to be the master of all. It is true that there is a communication overhead to adding more people so it will not scale linearly, but if a single developer has taken 8 years to build something in our era it seems likely that having 8 people might get it done say 1 and a half to two years.
