I spent a couple of days at a dixieland jazz festival that continuously had bands playing from morning to evening. It seemed that the more traditional bands that played the older material utilized tuba players, often sousaphones. The bands that also did some dixieland, but also played more relatively recent music, probably later in time than dixieland had the double bass players, who played more of a quarter note beat, or the bass player doubled on tuba. This crowd seemed to want that tuba sound for the really old tunes.
I sat in and played bass for some jam sessions then, and they loved the bass playing, but all the other players who signed up were tuba players, young and old. So, it seems tuba is very alive and well.
This was definitely an older crowd and tubas were often used, I would say in over half of the performing bands. But some dixieland bands have this kind of more modern sound, or they are playing more tunes from the jazz Real Books, have a vocalist vs. the traditional dixieland material. In those bands, double bass was used, and often the bass of choice was some kind of electric upright bass. The EUB players tends to play straight quarter note walking lines. I got the impression there is room for both tuba and bass in this genre of music.