Use of the apparent difficult has been proposed many times before-- it and other schemes that result in a unique value for each block which is calcuable by the block finder immediately lead to a withholding attack
The withholding attack also reduces bitcoin's stochastic finality: a tx 6 blocks deep still has a nontrvial chance of being reorged.
The scheme has other downsides as well:
Anytime two blocks are found in short succession, with the later one having a lower hash, it causes the earlier to be reorged, when that earlier one has already been relayed across most of the network,
wasting bandwidth and causing unnecessarily many small reorgs.