If financial modernization allows all financial firms to affiliate, but prohibits banking and commerce, the pre-existing commercial affiliates of nonbank financial firms would have to be divested to acquire a bank, while banks could enter de novo the new nonbank financial activity without divesting any valuable assets. This is either inequitable or the cost of acquiring a bank, depending on your point of view.
But, it is clearly a problem that has to be addressed. The choices are divestiture, grandfathering, long-term phase outs, basket clauses, or combining of banking and commerce. It would seem to me a poor public policy that opened up banking and commerce on pure equity grounds, rather than a disinterested analysis of societal net benefits.