You are misunderstanding that 6-month protection concept. They feel confident that the protection will last at least six months. It has not been around long enough for them to project further. But they are expecting that additional boosters will be needed.

As for the variants, you are absolutely right. They pose a threat, but no one can predict when the next variant will come along that renders vaccines ineffective. All they know is that the more unvaccinated people there are, the more cases there will be, and the likelihood of a dangerous variant will be higher.

Hope that helps.