The down side to the fiduciary rule is that if it goes into effect, people without much assets to invest won't be profitable clients, and won't get any kind of investment advice. That doesn't mean, by the way, that they will put it all into index funds instead, the kind of people who are so reliant on advisers that they don't know they are being scammed don't know an index fund from a hole in the ground. I have known many people who literally have no idea how one would buy a stock, or a mutual fund, or what the difference is.
Is the current system, with most financial advisers probably giving ok but not ideal advice because they get commissions, better or worse for the long term financial health of most people? I don't know of anyone who has analyzed it thoroughly, but I think there would be losers as well as winners if the fiduciary rule goes into effect.