This is literally how the Adaptive immune system works.
Initial adaptive response, production of IgM, Vdj recombination, class switching and global proliferation (especially in the face of a repeated antigen insult), followed by a tone down of B cells as the memory effect is preserved and the antibody titre falls back (antibodies have a half life of 3-4 weeks).
Because (at least currently) the antibody response in humans is non-sterilising (talk of boosters to generate IgA abound, along with a more specific Delta variant vaccine may change this) you can’t equate a titre in humans yet with a ‘level’ of immunity
Having lots of antibodies when exposed can maybe prevent infection or lessen severity, so it isn't a complete non-concern that they fade.
the durability of immunity may be inferred by observing the increased response to challenge over points in time.
So someone could (in theory) get the vaccine, have tons of antibodies 6 weeks later, then have greatly reduced antibodies and it doesn’t tell us anything about whether or how much immunity that person will have if they’re infected?