Suppose that a number of people form a group in order to obtain some goal, X. They appoint some of their members to be the leaders of the group. These leaders quickly realize that if goal X is actually achieved, the group will disband, and the leaders of the group will be out of a job. At the same time, if they refuse to work toward X, they will be fired. Therefore, the leaders of the group take actions that lead the group progressively toward X but make sure that X is never achieved -- a political version of Zeno's paradox.
Does this story sound plausible? Economists will recognize it as an agency-cost theory: the agent (the leader or leaders) and the principals (the members) have divergent interests. Both want to achieve X but the agent also wants to keep his job. If group members cannot adequately monitor his actions, the agent will take steps that best satisfy both his ends, which means working toward X but not actually achieving it. Because the group cannot distinguish between honestly working toward X and deceitfully taking baby steps, they cannot discipline the leader for his bad behavior.
The story underlies Jack's claim that the leaders of the Republican Party don't really want the Supreme Court to overturn Roe, because once that happens, they will be out of the job. They have been working toward X for 35 years now, but they will make sure never to achieve the goal, apparently by ensuring that people willing to overturn Roe are not appointed to the Supreme Court. David tries to refute this argument by adopting its logic to gun rights. The Republican Party also seeks to secure guns rights. If it succeeds, then it will fall apart. Yet the Supreme Court justices seem inclined to deliver success. This must mean that Jack's logic is faulty, the leaders of the Republican Party actually do want to overturn Roe, and hence Roe will be overturned. Jack responds by denying David's implicit premise that establishing gun rights serves the Republican coalition in the same way that overturning Roe does.
Jack's theory can be attacked more directly. The agency theory assumes an information asymmetry between leaders and members: the members cannot monitor the leaders' behavior. In many settings, such an assumption is reasonable, but here it is not. It is perfectly clear what the Republican Party leadership is, and is not, doing to undermine Roe. That they have not put all their resources into this task is consistent with an innocent explanation: they have to please all members of their coalition and these people care about things other than (or in addition to) overturning Roe. If Bush had put all his political capital (when he had political capital) into appointing clearly anti-Roe jurists to the Supreme Court, he would not have been able to use it to obtain other things that he and other Republicans care about (tax reductions, etc.).
The theory also makes too much of the difference between overturning Roe and chipping away at it. Members of the Republican coalition are not so much concerned about Roe as about reducing the number of abortions. They also care about other things. So as the Roe precedent becomes weaker, it will become easier to restrict abortion, and therefore people opposed to abortion will feel decreasingly concerned about that issue and correspondingly more concerned about the other issues that they care about. If Jack's theory is right, then the leaders of the Republican Party can't merely preserve Roe as a hollowed out shell; they must ensure that, in fact, little progress is made in restricting abortion as a practical matter. In other words, they maintain their leadership by accomplishing very little for their followers, all the while arguing that they are doing their best. Not a plausible recipe for political success.