If you’re an optimist, or a Biden supporter, the killing of al-Zawahiri proves that it was unnecessary to keep American troops in Afghanistan, and that the administration’s decision to withdraw was correct. If you’re a pessimist, or a GOP voter, you probably think that the episode proves the Taliban will continue to harbor terrorists, and that an attack on American soil is inevitable as a result of the withdrawal.
Both arguments are plausible. The only way we will get a definitive answer is if the worst fears of the pessimists are shown by events to be correct. The Biden side has the obligation of proving a negative, which isn’t really possible; if there are no terrorist attacks for ten years, how do we know one isn’t just around the corner?