On the flip side, I've seen a project fail because it was built on the unvalidated assumption that the naive architecture would scale to real world loads only t

On the flip side, I've seen a project fail because it was built on the unvalidated assumption that the naive architecture would scale to real world loads only to find that a modest real world workload was exceeding targets by a factor of 100X. You really do need technical leadership with good judgment and experience; we can't substitute it with facile "assume low scale" or "assume large scale" axioms.