Well, I'm going to be the one that points out the nature of the emperor's suit.
The use of the term "arm weight" is bogus for the majority of bass players who play bass in a near-vertical position. In that position, gravity pulls your arm and hand down toward the FLOOR, not back against the fingerboard. As near as I can tell, what's actually meant is "pull back against the fingerboard with the big muscles in your upper back." OK, that's fine; but I don't know how "weight" got into it. The terms "weight" and "gravity" have actual meanings and "a force exerted by the muscles of the upper back more or less parallel to the surface of the earth" ain't it.
Now let's talk about pulling back with the muscles of the upper back. In lower positions (physically higher up, further from the floor), this ain't gonna happen either. Why? Well, because if you pull back against the fingerboard at the top of the neck, one of two things will happen, maybe both at the same time. 1) Because the bass is only constrained in the middle where it presses against your belly, and at the floor where the end pin (rubber foot, usually) sits on the floor, the thing's gonna wave all over the place in the breeze. 2) Because you're pulling backward at the top of the neck, the end pin is going to be pushing forward against the floor (anyone ever hear of Newton's Laws of Motion - equal and opposite forces?) and if it's not a new real sticky rubber foot, it's going to slide out from under the bass. So what really has to happen in low positions is that you're going to have to exert force with your thumb to - at least partially, or mostly - counteract the forces applied by your fingers. Yeah, you can put a little force on your fingers from the upper back, reducing the force exerted by the thumb, but get too excessive here and the bass is going to wave all over the place.
What really happens, in my opinion, is that experienced bass players apply considerable force with their thumb on the back of the neck but they've built up the necessary muscles (the thumb is very strong in that direction) and they don't put any more force than required. So they're playing efficiently; but the gravitational force of the earth is not involved to any meaningful extent.
Now in the higher positions, the vertical distance between where your belly touches the bass and where your hand is, is much reduced so the resulting reaction force at the end pin is correspondingly much reduced; and then you really can get the muscles of the upper back into play. And if you play sitting with the bass fully constrained and more nearly horizontal, then the gravitational force of the earth can assist in stopping the strings. But for normal near-vertical playing and in low positions, I respectfully submit that "arm weight" is a gross misnomer.
I would dearly love to put a force transducer under the thumbs of some expert players who claim they play without thumb force and see the actual measurements, playing let's say a bunch of half position stuff in the key of F or Eb. I bet you'd find that if it takes 15 lbs to stop the string, they're applying 1 lb with the muscles of the upper back, 14.001 lb with the thumb, and not an ounce more than needed.