Just about everything in a band situation is negotiable.
You have to go in knowing what your standards are. Get crystal clear with yourself about your goals & expectations. Then communicate that clearly & precisely to the others in band. Ask them to do the same. Figure out what they want from being in the band. What they are willing to do or not.
Sometimes you have to figure out these things on the fly as a situation develops. But it helps if everyone already has a clear understanding of what they want, what others want, so everyone knows what to expect.
For this particular situation I can see it going either way. It just depends on what it's worth to each person.
Which one has more value for you?
A. A working band to play tunes, but the bass player will play whatever he wants & not necessarily the existing bass line.
B. A band looking for a bass player that will play the simple existing bass line.
Either one could be acceptable to any one band member or group of bandmates. Each of you has to make that call based on your individual standards.
When I've been in situations like this it is usually a negotation. I'd ask the other member (bass player in your case) exactly what they intend to play & then assess it. Is it in key? Rhythmically & melodically musical? Are they aware of the overall dynamics & story arc of the song? Will they play more actively during the instrumental section & simplify when it might interfere with the vocals?
In other words: is what they are going to play integrated with the song? If it's not, then explain why & work with them on how to fix it. If they refuse, then it's time to part ways. No hard feelings. Just a matter of goals & expectations that don't align.
Best of success with your bass player!
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