Just to be abundantly clear, the current COVID vaccines are very effective at combatting all currently-known variants of the disease, and there is no evidence that the effectiveness has waned over time for any given variant for anyone that has actually been given the vaccine. We believe boosters will be necessary because a) the existing vaccines are somewhat less effective against some new variants and we believe that eventually a yet-to-be-discovered variant will create a significant reduction in effectiveness (especially given the number of unvaccinated individuals who will be hosts to mutating forms of the virus), and b) we have seen clear evidence that natural immunity from COVID infection (but not necessarily vaccines) wanes over time, so we believe that the vaccines will eventually do that too.
That is to say that the need for annual injections or other forms of boosters is likely given the available evidence, but is not a foregone conclusion (and would likely be mitigated significantly with less vaccine hesitancy).