Take this with a pinch of salt. With consultants it's YAGNI until you do need it, at which point you will be billed at their contingency rate! You don't need to go full waterfall to understand your requirements up front and make the choice not to wait until something becomes a showstopper because "YAGNI and it won't fit into this sprint anyway". Trust your experienced, in-house engineers for what you are or aren't gonna need.