One of the interesting things about memory B and T cells is that there is no way for the body to know whether they will be needed again. Some studies have shown that they become a somewhat permanent resident of the human body. For instance, people that are vaccinated by smallpox still have memory cells 30 years or more after the original vaccination. In this case, clearly a smallpox memory cell isn’t needed because smallpox as a virus is eradicated (extinct). Though, there is no way for the memory cells to know that the smallpox virus doesn’t exist anymore so they’ll persist in the human body.

So the idea of the memory cells is that they may never be needed again, but they’re there as a backup in case the pathogen that they recognize returns. Individual memory cells probably don’t live 30 years and almost certainly divide and multiply to keep their numbers stable. In this case, old memory cells would undergo apoptosis and die. However, they would leave behind an identical memory cell that recognizes the same pathogen. So the response to pathogen by a memory cell would be the same except it may be by a descendant of the original memory cell.

I think they stick around, but in low numbers, in case that the pathogen comes back and they are needed again.