The thing with natural immunity is it can last anywhere from 2-3 months to years depending on the viral load of the initial infection and your personal immune system. As it's so hard to measure T cell responses, we can't tell...so we have to assume it's 2-3 months for everyone. Yes, it's possible some people may get lifelong immunity...some SARS-CoV survivors had partial protection against SARS-CoV-2...but it's too much of a gamble.

Additionally, while it seems that protection against MILD infection has faded with Pfizer after 5-6 months, protection against hospitalization is solid. Moderna seems to last a bit longer.

And a third booster could induce more durable protection. It IS possible that Fauci is right and we really need a three dose series. There's at least one childhood vaccine that's 1 shot, 1 month, 2nd shot, 6 months, 3rd shot...it's one of the Hepatitis shots but I forget which one.