This is a very old idea which has been discussed to death.

People joined Bitcoin knowing that bitcoins would gradually be lost over time. This is an important part of how the Bitcoin currency works. Therefore, reassigning "lost" BTC would be extremely similar to increasing the money supply beyond 21 million, and so it is an absolutely prohibited change. Nothing that does this can ever rightly be called Bitcoin, no matter how much consensus it has. If we ever run low on satoshi, the solution will be to increase precision, not to add more satoshi.

Expiring very old UTXOs but not reassigning their value is acceptable, though. In fact, this can be done in a softfork. I think that doing this would be a good idea:
- If apparently-lost BTC is at risk of being recovered illegitimately due to cryptographic weaknesses. The release of so many lost coins would be a major blow to the the economy. Once it looks like ECDSA will only be secure for 5 more years or so, I think that a timer should be started to expire all ECDSA-secured UTXOs at the expected point at which they will become too insecure.
- If storing the UTXO set becomes a very major bottleneck for full nodes and there are no other good alternatives to UTXO expiration on the table. Several years of advance warning should be given before the first expiration.