What happens when there are no descendents left to pay for cemetery plot maintenance?

I went with a friend of mine to visit her deceased parents’ gravesites. She told me that she paid years ago $500 and the interest from that pays for the monthly maintenance of the ground around the plot. She said the alternative would have been to pay on a monthly basis. I asked her what would happen if there were no relatives surviving that could pay that? She said “there would have to be somebody that could pay it.”

Obviously, there should not or could not be some plots where the grass grows out of control while others are maintained. What happens when there are no surviving relatives left to pay for plot maintenance? Why would people who survive continue to pay when the whole cemetery is maintained regardless of people paying or not paying?