I think you are the golf ball balancing perfectly on an upside down bowl. Optimal, but an unstable solution. Most engineers don't yet know because they don't have enough experience (and most engineers haven't worked in high performance for most of their career), so they need the benchmarks and profiling.

Plus, benchmarks are good solely to be able to show your manager that yes, spending 3 weeks on that refactor was indeed useful. Engineers shouldn't need to have to do that, but it is often useful none the less.