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. I said it's contrived as anything, but it's all technically possible if you mess with the incentives and intentionally create really perverse systems. That's the messiest of them though, changing the transaction format, script language, issuing new coins or whatever is pretty clean in comparison.
> 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.
You can remove the block reward as a soft fork, the consensus rule for Bitcoin and its derivatives is a maximum. Even if it wasn't, you can soft fork in a rule that the block reward should be spent to somewhere invalid, or paid to a particular person.
That doesn't work, unless 100% of miners accept your rule change forever (aka impossible). It would only take a single miner to keep mining using the old rules and this miner would reap the block reward / transaction fees. An old Bitcoin client would follow the chain created by this miner, wouldn't it?
