It's an opinion and it brings up some valid points, but there are flaws in any approach and Volcker rule is not an exception. However, there is certainly evidence that when the banks were allowed to warehouse risks, they have fucked it up big time (having worked as a prop trader at multiple major IBs, I've witnessed it first-hand).
The general logic is that the banks are systemically important. Preventing them from directly warehousing risk does probably decrease liquidity, but that's the cost we (as an economy) are willing to bear to avoid bank failures. Pretty much any other approach like regulatory capital, systematic risk stress-tests etc would have very similar implications.