they're not sure if the vaccine will last in your system for more than 30 years
Vaccines don't really last in your system more than a few days (although some cells get dragged off to the lymph nodes and get harshly interrogated for longer than that). It is the T-cell and B-cell responses that are persistent for years.
and that subsequent doses may be less effective
They should activate B cells that you already have and produce high-affinity mature antibodies.