I think the flaw in the syllogism is “the human reason, unassisted, has a low chance of retaining its hold on truths.” We certainly forget a great deal of procedural and propositional knowledge if we don’t use it on a regular basis, but that’s different from letting go of a belief because you are passionate about how inconvenient the belief is. Once a belief takes root—i.e., after you announce it to your friends and take some actions based on it—it is usually very difficult to let go of that belief.
I think the flaw in the syllogism is “the human reason, unassisted, has a low chance of retaining its hold on truths.” We certainly forget a great deal of procedural and propositional knowledge if we don’t use it on a regular basis, but that’s different from letting go of a belief because you are passionate about how inconvenient the belief is. Once a belief takes root—i.e., after you announce it to your friends and take some actions based on it—it is usually very difficult to let go of that belief.