What happens to our coins when an old privacy pool is no longer supported?

Old systems can become hard to maintain, and sometimes they need to be replaced. I have no problem with that. But I do have a problem with what happens to the people who still have money in those old systems. Imagine someone who used Zcash many years ago. They received some ZEC in a Sprout address, kept their private key safe, and simply left the coins there. Maybe they forgot about it for a few years. Maybe they did not need to move the coins. Now the network moves forward, and that old privacy pool is no longer supported. The coins are still there, but the owner can no longer spend them. I find this hard to accept. If I put money in a bank, I don’t expect the bank to tell me a few years later that my account is no longer supported and that I can no longer take my money out. I know a blockchain is not a bank, but the basic idea of owning something should not be that different. If I have the private key, I should have a way to prove that the coins belong to me. So I started thinking about a possible s