I can't speak for Evan above, but I do agree with him that your understanding is flawed (could be for different reasons, I don't know his). We don't know exactly how long the vaccines will provide immunity, but it is likely longer than 6-8 months. Natural immunity likely provides years of immunity, at least to some degree, so there's no reason to think the vaccine won't provide that long either. Even if you get it after the vaccine (say a year or two after), you are likely to have a less severe case of it. Could future booster shots be needed? It is possible, but even for the people that need them, they aren't in the same spot as an unvaccinated and previously uninfected person.
One thing that worries me going forward is how people, particularly media and certain parts of the over-zealous medical community, treat future cases of COVID. COVID is never going away completely, zero-COVID is a fantasy. It's going to be in circulation this year, next year, and 50 years from now. Between vaccine immunity and natural immunity, we should hit herd immunity and see the spread be much less, and in future winters the severity should be less because of multiple years of exposure to both the virus directly and the vaccines. Due to this, it'll likely be no more severe than the flu (or perhaps less severe than the seasonal flu) within a year or two. If the efficacy numbers from the vaccine trials hit anywhere near that in the real world, it could certainly be much less severe than the flu overall. Once we hit that point, will people continue to treat it the way they do now when cases start to increase? Coronavirus OC43 is thought to have caused a very similar pandemic to COVID-19 in 1890 (albeit unproven, some research suggests OC43 caused the Russian Flu pandemic of 1890), and it's one of the common causes of the cold today. We track it to some extent, but nobody knows or cares outside of certain parts of the medical community. I hope that once we turn that corner that COVID won't be much more than an afterthought, but I'm not optimistic that that will be the case.