There was a news item in the past year or two about brokerage fees in the US for IPOs. The spread on fees was ... distinctly narrow. Especially as compared with other markets (EU?). Strongly suggesting collusive / oligopolistic behavior.
Unfortunately, this is a market with a high barrier to entry, since IB's reputation for many investors is a proxy for the quality of due diligence. It's hard to disrupt this market unless companies will be willing to do their IPOs in other markets, like Europe or Asia.
In the past, when a market has tended toward a monopoly, the usual solution hasn't been to create new entrants so much as to split the existing monopoly.
Though both approaches have been tried, I suppose. Microsoft would be an instance in which an illegal monopoly (read the court findings, folks) was not split, as many commentators suggested, but regulated. It's arguable that this did result in a loss of standing as a monopoly, though how much of this is a matter of a shifting computer market (servers, mobile, cloud) and how much an erosion of Microsoft's still overwhelmingly dominant share of desktop OS and office suite markets is also of interest.
NB: I think Microsoft should have been partitioned, and that it still behaves as if it had and seeks an illegal monopoly, though its power has been greatly diminished.