Speaking specifically to the Pfizer vaccine, it’s gone from 95% effective against preventing severe symptoms against the Alpha variant to 88% against Delta in less than 6 months of the vaccine being widely available to the public (With some even less optimistic peer-reviewed studies coming out of Israel, I’m just going by what the CDC is reporting). So under these circumstances, maybe it makes sense to wait a year or two before making claims about the long-term effectiveness of the vaccines. If they aren’t effective long-term some people might make different decisions about what vaccine they decide to take.