It would seem like it would depend on what you are planning to do with that extra space.

So, say you're building a steel bike with standard tubing sizes, and welding on a 68mm bb shell, or a 100mm bb shell, then the forces on the seattube and downtube would be the same, assuming you would get your cranks and pedals in the same place.

Using narrow crank spacing, plus some sort of pedal extenders or long pedal spindles has an advantage of reducing crank rub (which might be a problem for those wearing size 15 shoes).

MTB cranks may already have somewhat wider Q-Factors than Road cranksets, but it would also be easier to build one frame & BB, but multiple lengths and Q-Factors of cranksets.

Perhaps have the Q-Factor proportional to the crank length. So, 170mm cranks, narrow Q-Factor. 180mm cranks, a little wider Q-Factor, and 200mm cranks, even a bit wider of a Q-Factor.