When the last crisis kicked off, organisations such as Goldman Sachs were not banks, and therefore would not have able to request support from the US government had the proposed Volcker Rule been in force then. In fact they weren’t able to ask for government help then either.
So how did the US Government respond to the crises at Goldman, Morgan Stanley and the rest? By making them banks, even though they plainly were not. It was a fig leaf, to bend the rules so that they would not fail.
So if, in order to avoid the Volcker Rule, these same organisations ceased to be banks, and if there were a new crises that threatened them, what would the US Government do? Going by the evidence of 2008, they would allow them to be banks again.
What possible good will the Volcker Rule have done then, other than to create an illusion of safety for the government and allowing the investment banks to engage in dangerous activities until things get too hot, at which point they will be rescued just like they were last time?