Hey GL; don't know if you got your answer yet, but, here's my $0.019:
First, Leo started with that design, it remains popular, and business folks don't change successful products.
Second, there are some playing styles that do benefit from 20 frets. Slapping (a misnomer, should be thump and snap, but that doesn't sound as good LOL) in particular benefits for thumping as close to the middle of the stopped string length as possible (a la Marcus Miller). You can execute the technique on basses with more frets, but the sound is different as your right hand gets positioned closer to the bridge. So, many players prefer the 20 for that reason alone.
IME, 20 is more than sufficient for standard cover gig material. You can also get 21, 22, and 24 on the various Custom Shop , Deluxe, and Fender derivative designs, though; so if you play parts in the upper register, that is also available. You can also get a 5 and string it E to C with a HipShot on the E. There are many options now. I do like 24 frets on a 4 if I am playing heads for Jazz standards, like Take 5.
Hope that makes sense. There is no perfect bass one way or the other. Trade offs and depends on the gig. YMMV