I'm hesitant to call that a proof, there are various points in the proof that might not be false, but are certainly non-obvious. Firstly, it seems to be enough

I'm hesitant to call that a proof, there are various points in the proof that might not be false, but are certainly non-obvious. Firstly, it seems to be enough for any alternative consensus to be impossible to maintain, rather than impossible to produce. Secondly, to make it practically (economically) impossible to maintain an alternative consensus should requires Proof-Of-Resources, not Proof-Of-Work. Thirdly, there's no reason to assume those resources can't be stakes in the consensus itself. Finally, if the only way to maintain consensus is by building massive expensive specialised buildings, then congratulations you've just rediscovered a bank.