The idea that there is one set place for the thumb requires the idea that the relationship between a thumb and the rest of its hand is always the same. My fingers are medium length at best, but the thumb seems to have come from a different set of gauges.
Except when playing a neck with a very deep profile (eg a BB424) my left thumb has to be allowed to wander around, keeping itself out of trouble as best it can. If I park my right thumb on the pickup in the classic prissy white boy style, my first two finger scarcely reach the E string, let alone the rest.
So either I have to walk away from bass playing, or the uptight notions of bass playing taught by the Neasden University of Music (formerly World Of Carpets Brent) can do one.