I don't think it will ever happen. But if it does, you can be sure that within about 10 years of that "law" or change, plenty of coins will move from old addresses to new ones, then they are no longer considered "abandoned".

But, for all coins that reside on addresses and not public keys (P2PK), on any address that begins with 1, 3, or bc1, those will never be spent unless someone has the private key. No bitcoiner will agree to that, the nodes, the miners... mostly the nodes. It will be considered a hard fork and most likely bitcoin will remain bitcoin and bitcoin-spend-anyone will die a horrible death or state of existence similar to bitcoin cash or bitcoin SV. (which means no one is using them unless they want to store weather data or some garbage.)