Well, if Bella were dead, then she couldn't be her kids' mom. Rosalie would remain an aunt, but a primary-caregiver aunt instead of an occasional-caregiver aunt. Less competition for the child's attention.

Edit: And it's not like the only reason Rosalie considered offing Bella was for egg-related reasons. Remember, Rosalie doesn't like being a vampire, she would have wanted someone to kill her when she asked. Bella's situation is different - better - in several ways, but insofar as Rosalie understands Bella by analogy to herself, it seems like it might be in Bella's interests to die instead of turning.