Warren in Lancaster, PA
The friction forces on a cap iron are dwarfed by forces we can both notice and easily do something about. For example, poor placement of the cap iron can easily be felt, but many just set the cap iron extremely close and don't bother to look for an ideal.

Another example is dullness. David did experiments to see how long an iron can go before it stops cutting. However someone who is seriously planing wood figures out that after not much dullness the increased force encourages sharpening.

I have seen videos of well known craftsmen honing at 38 to 45 degrees. This is another example of a force that can easily be discerned and can be easily be lessened.

I have more examples, high angle planing, too thick a shaving, heavy plane, on and on. But the point is to focus on forces we actually notice.