One more piece of this is they endlessly mutate, I don't think in reality there is a limit. Most mutations probably don't help the virus proliferate, might cause it to not be effective. Eventually the shape of a new one, what your body is looking for in it's remembered virus history will change enough that it can't be detected. We see this in real time with the cv19 variants. My mental model of this (I am in software so I welcome corrections to the applicability) is it's like antivirus in software - the bad guys making viruses keep trying new variations on old ones, switching what the protection software is looking for so they can get around detection methods, as well as trying to find new holes.
The lead time for pushing new "virus definitions" is a lot shorter once the vaccine is adopted and manufacturing chain is established. A central lab can download the new variants genome from labs all over the world, select some markers and push changes to manufacturing facilities through e-mail or even Git.
