IEX is an ATS. The ECN/ATS business is dominated by rampant and well known conflicts of interest. A part of the IEX value proposition from the beginning was to offer an alternative to traders who were disenfranchised by this market structure.
If maker-taker rebates are part of your trading business model or if you engage in any strategy that could be deemed latency arbitrage then their value proposition is deliberately adversarial to your goals, because they're competing to take flow that feeds your strategy. They are specifically for the trader who wants to route through a different market structure.
If IEX routes deliver on the value proposition then they could potentially be more profitable for automated strategies around threshold level-one book liquidity levels who want to avoid having portions of their order slipped to worse pricing through latency arbitrage. As IEX increases their flow, their ability to deliver in this department should also increase. But they can also do this to the extent their algorithms are good at matching against external flow without allowing latency arb bots time to react.
Your analysis of the value has to focus on pricing of fills versus the costs you pay for the routes. There are many automated traders for whom latency arbitrage induced slippage could handily exceed the price difference on routes which are cheaper than IEX. Of course, this is all subject to how well they deliver in terms of order fill. And it's hard to get good analysis in a world of vendors who will only find statistics that support you giving them flow. If you haven't caught some of these guys lying then you haven't been paying attention. Once again, IEX is aiming for something different by unique transparency.
If you're saying that other ECNs/ATSs inflicted the competition from someone like IEX upon themselves, I think that's a fairly reasonable analysis of how competitive forces work.
Sorry if this answer uses too much jargon. I expect a lot of stackexchange users. But do feel free to ask in comments if I could provide more clarity.