So McCorvey wasn't the best plaintiff to pick, in hindsight....so what? There were plenty of women who were denied abortions who could just have easily have been plaintiffs, and there are plenty of women today who would line up around the block to be plaintiffs to challenge anti-abortion laws if more are passed. You'd have to beat them off with a stick.
The Supreme Court did not decide Roe based on the particulars of the plaintiff before it, i.e. whether or not McCorvey was a stable, sober person. It decided Roe based on constitutional principles. McCorvey's past, present, and future have absolutely nothing to do with the legal basis of the decision. At the time, she had standing to pursue the suit, and that's what got the case in the door in order for the courts to have jurisdiction to adjudicate it. In other words, she was able to demonstrate an injury in fact that is actual and concrete, not remote and speculative (i.e. an unwanted pregnancy that she was unable to terminate legally) directly traceable to the complained-of conduct by the state (i.e. the anti-abortion laws) and redressable by the court (through a decision overturning the anti-abortion laws). Her opinions on the matter in hindsight are irrelevant.
To challenge a law, a plaintiff must currently have standing. So, for example, if new anti-abortion laws are passed, women who currently are pregnant and cannot get safe and legal abortions (or doctors who provide abortions, another common class of plaintiffs in these types of suits) will have to be the new plaintiffs. Conversely, if a law guaranteeing the right to safe and legal abortion were passed, any anti-abortion people who wanted to challenge it would have to demonstrate standing of their own. Once plaintiffs are in the door with standing, the court then decides the constitutionality of the law before it. Whether or not a plaintiff changes her mind later about an injury she may have pled at the time of the suit doesn't matter if the suit is over and done with.