Yep, last night I went to our local week long music festival ( Festival d'été de Québec ) where we have big act all week and some local act. Yesterday I saw a French act called "M" and he didn't have a bass player. Nope, instead he had another guitar player with many pad and synth stuff attach to the guitar body to trigger varous effects. That guy was also playing the bass with his guitar ( the E and A stringss where bright yellow while the rest were normal ) so he had some kind of octave effect that only worked with the E and A strings. Quite clever actually. So I think my point is, there is no instrument that can't be replace by something else.