>snip< all appear to have a bottom yoke substantially thicker than the top yoke, with up to four pinch bolts on each side, clamping the fork legs. A couple of simple force diagrams show that a stiffer bottom yoke imposes much higher loads on the fork leg, ultimately to the point where the top yoke takes no load and the full bending moment applied to the leg is taken out over the thickness of the bottom yoke.<<
Even if the lower clamp has the necessary KILLER grip on the tubes, I suspect the lower clamp's grip on the stem would slip when subjected ~1000 lb-ft moment of heavy braking. Then some of it will resolve back into radial loads of opposing sign at the upper and lower bearings via the tubes.
Isn't the moment the same at the lower clamp whether fixed or simply supported? When I bent my 32 and 35 mm dia forks (with wimpy single bolt lower triple clamps) by panic-ing smashing into things, the fork tubes bent at the lower triple clamp, where the moment was highest, which would seem reasonable, even if the triple clamps were holding the tubes like simple supports.
One time I time I bent the triple clamp also, suggesting its single bolted grip was stiff enough to force it to transmit some moment.
I think the modern lower clamp's beefiness has 2 values.
1 is to better pass the axial nearly vertical loads into the lower head bearing without tipping or "working" the clamped joint. I think of the lower head bearing applying a force to a cantilevered beam "fixed" at the fork tube. Then, Just as you suspected, the multiple widespread bolts can provide the "fixing."
2 - Multiple clamp bolts in the lower clamp also help keep the tubes from twisting and shifting alignment. When viewed from the side, going from || to X. Maybe good for smooth fork action, and forcibly steering out of off-road ruts, among other things.
If the forks were vertical (rake=0) with no offset in the triple clamps (o-o-o) the lower steering head bearing would take ONLY axial upward from weight, and side radial loads. THe upper bearing could take ONLY the occasional axial downward (dangling fork weight/inertia , as from wheelies, jumps, and potholes) and radial loads. Braking forces would resolve into opposing radial loads at the upper and lower bearings via the fork tubes.