I think the question assumes that the bike fits perfectly in every other way. If that were the case, I might be somewhat inclined to agree.

But fit almost always a compromise. And even in the same frame size, you'll have varying lengths of top tube among various brands. Plus, if you throw in head tube and seat tube angles, and the math quickly becomes impossibly complicated, except maybe for the pocket-protector, slide rule types. And how many of them are serious cyclists?

Another factor is weight loss. A smaller gut will allow a more stretched-out position to be comfortable than was ever possible with beer gut firmly in place.

Also relevant is how long the rider will be on the bike. What's comfortable for a one hour loop through the countryside, will be different from what would be best to use for competing in the Race Across America. Or a triathlon. Or a criterium. Or a trip to the store.

I think the #1 factor ought to be comfort, but a bunch of factors come into play there as well, as what's comfortable for one person with long arms and short legs, won't be comfortable for someone else with short arms and long legs. Even if their height and weights are identical.

So taking all of this into account, I don't see how any bike designer can say definitively what size stem a given frame size should properly use. It's all about compromises and balancing this factor vs. that factor, and hoping that the result of all this number crunching results in something that "feels right". Which is a completely subjective determination in itself.