I'm open to an alternative explanation, however it seems
Well not entirely, you have to consider that fender are a huge corporation like Ford or Toyota. It makes sense for them standardise the fretboard to 20 frets, if that's what the customers want and it's cheap. It's too risky for them to change it now, They'd have to change the body to match and that might turn into a 'new coke' fiasco. But I'm open to an alternative explanation.
I'm fairly anti-corporate and skeptical of capitalist philosophy and rationales. But I also don't buy into the socialist interpretation of everything being driven by greed and self interest.
I do seem to recall Leo saying in an interview (I'll have to see if I can get a citatiion for it) that the 20 frets seemed to him to have been enough for the needs of bass players in the era the PB and JB were created. Possibly because, with the heavy guage flatwounds of the period, there were diminishing returns in putting more narrowly spaced frets higher up in the neck? And his primary goal was to make a decent sounding but still affordable instrument.
As the playing styles and tastes in bass changed, his subsequent companies may have done things differently in reponse to customer demand. But that said, most players I know seem to hang (at least from where I'm sitting) between the third and twelfth frets 99% of the time.
I'm not a particularly big fan of the Fender bass sound myself, although I do have a good deal of respect for it. Suffice to say it does the job and has its share of admirers. So why Fender would want to change that just to address something not many of its customers are clamoring for seems silly to me. And to your earlier point, there already are the new generation JB lookalikes that address those issues (for some players) that Fender doesn't see fit to address itself.
I think that's more driven by what its actual customers want than by greed, cost cutting, or their clientele being ignorant sheep mired in the past.
Part of mark of the true master is knowing when your piece of art is finished - and having the confidence (and discipline) to put down your metaphoric brush and say: "It's done."
In the case of the classic PB and JB, I think it's more a case that they are "finished." And Fender wisely has chosen not to gild the lily, and therefor continues to produce them virtually unchanged.
P.S. - I apologize for the use of the word 'ignorant' in my earlier post. What I was trying to say was I felt your comment glossed over a few things and mischaracterized where (I think) Fender is coming from with regards to their mainstream offerings.
(Type something in haste, repent afterwards and have to apologize for it. I should have learned that lesson by now.)