Billy's also the only prominent bassist I've heard who really incorporates some of John's signature phrasing in his playing. He's got his own thing going, but the influence is clearly there.
I think that Pete and Rog don't want to try to fill John's shoes with a virtuoso player, just like they didn't try to replace Moon with another wild drummer. Jones was a timekeeper, Phillips was creative-yet-disciplined, and Zak can imitate Moon's parts and style without overwhelming or grabbing too much of the spotlight. They're around 60 now, and I think Pete wants to close things out with as much of the spotlight on his guitar playing as possible. He's come a long way on lead guitar, and now he's got instrumental center stage all to himself.
Also, neither Pete nor Rog ever used flashy bass players on their solo records - creative and interesting yes, but virtuosos and showstoppers - no.