> Nobody would be mining OldChain so it's difficulty would be irrelevantly close to zero, you'd just just drop the requirement for POW down to something effectively gone.

Why would nobody be mining OldChain? You can't remove the block reward or change the difficulty without a hard fork. PoW miners would still decide which blocks are mined and reap the rewards (block reward and transaction fees). You haven't actually proposed a solution that wouldn't require a hard fork.