> All we know is that an additional dose does increase your protection, but it doesn't necessarily give you a decade of protection since there were people who got the 3rd dose and still got COVID.

That doesn't mean anything regarding protection longevity. There will always be instances of individuals who fail to receive immune protection from a vaccine. The vast majority of people could potentially receive years of protection with a 3rd dose, while a small group doesn't get any protection at all. That isn't specific to covid.