Degenerative spondylolistheses can degenerate further and become unstable but worrying about that is somewhat like worrying about getting hit by a meteor when you walk outside. It most likely will not happen but if it does, you will know it. Don’t waste your energy worrying about this possibility.
Actually, the flexion is more of a problem for the disc bulge and extension is more problematic for the slip but again, don’t be too concerned about that. Core strengthening is the key to stabilizing the spine.
I cannot tell you how many times a degenerative spondylolisthesis needs surgery as the population I see has enough symptoms to consider surgery. I have no doubts that there are many more walking around with this diagnosis who don’t even know it.
Most patients with this diagnosis do not need surgery so the answer to your last question is yes.