You're misinformed, the "underlying tech" of the flu vaccines is absolutely not "the same." And this isn't even an issue of them being updated, none of the major vaccines have been reformulated yet, it's a question of what's going to happen as more time passes, the virus mutates further (possibly in reaction to evolutionary pressure from the vaccines themselves), and antibodies from the initial doses start to wane.
One possibility that both the FDA and Fauci have acknowledged is "disease enhancement," i.e. the vaccines actually resulting in more severe illness over time. We simply haven't seen them in the wild enough to rule that out.