Actually I mentioned a proof of space and time hybrid which is much
 different than staking. Sorry to draw for the confusion as PoC is more 
commonly used then PoST.
It
 has rarely been done though given the technological complexity of being
 both CPU compatible and memory-hard compatible. There are lots of 
benefits outside of the realm of efficiency, and I already looked into 
numerous fault tolerant designs as well and what others in the 
cryptography community attempted to propose. The actual argument you 
have only against this is the Proof of Memory fallacy, which is only 
partially true. Given how the current hashing algorithm works, hard 
memory allocation wouldn't be of much benefit given it is more optimized
 for CPU/ASIC specific mining. I'm working towards a hybrid mechanism 
that fixes that. BTW: The way Bitcoin currently stands in its 
cryptography still needs updating regardless. If someone figures out NP 
hardness or the halting problem the traditional rule of millions of 
years to break all of Bitcoin's cryptography now comes down to minutes. 
Bitcoin is going to have to eventually radically upgrade their 
cryptography and hashing algo in the future regardless. I want to 
integrate some form of NP complexity in regards to the hybrid 
cryptography I'm aiming to provide which includes a polynomial time 
algorithm in the cryptography. More than likely the first version of my 
BTC hard fork will be coded in a way where integrating such complexity 
in the future only requires a soft fork or minor upgrade to its chain.
In regards to the argument, "As a separate issue, proposing a hard fork in the hashing algorithm will
 invalidate the enormous amount of capital expenditure by mining 
entities and disincentivize future capital expenditure into mining 
hardware that may compute these more "useful" proofs of work."
A
 large portion of BTC is already mined through AWS servers and non-asic 
specific hardware anyways. A majority of them would benefit from a 
hybrid proof, and the fact that it is hybrid in that manner wouldn't 
disenfranchise currently optimized mining entities as well.
 There are other reasons why a cryptography upgrade like this is 
beneficial. Theoretically one can argue BItcoin isn't fully 
decentralized. It is few unsolved mathematical proofs away from being 
entirely broken. My goal outside of efficiency is to build cryptography 
in a way that prevents such an event from happening in the future, if it
 was to ever happen. I have various research in regards to this area and
 work alot with distributed computing. I believe if the BTC community 
likes such a proposal, I would single handedly be able to build the 
cryptographic proof myself (though would like as many open source 
contributors as I can get :)
Anyways just 
something to consider. We are in the same space in regards to what 
warrants a shitcoin or the whole argument against staking.
Best regards,  Andrew