Elite RBs have been part of a many of Super Bowl teams. Chicken or egg? I think more is more. If Singletary turns into a dominant back, sign him.
The better the QB, the less dire the need of a superstar RB, but also the less dire the need of a superstar #1 WR or an all star OL or a Legion of Boom defense. You build a team with what ya got. A superstar QB mitigates just about every weak positional group on the entire team, but football is still a team sport, except each team may be structured differently depending on what resources are available in FA and how many hits ya get in the draft.