I basically agree with you, and the market has been deciding; IEX is currently the #2 ATS (colloquially "dark pool", but IEX has been publishing quotes). However, now that IEX is an exchange, their quotes become "protected", and all market participants are legally required to send orders to IEX if they're displaying the best price. It's now illegal to decide that you value a quick response or a higher chance of getting a fill over price and choose not to trade on IEX.