Yes, you may be correct in one sense. But "your" market is not a "functional" one in the sense that the National Best Bid and National Best Offer are illusory then. ..........
I have explained these ideas years ago, but this is certainly true. NBBO must be accurately time-stamped and distributed to all parties equally, or there is no such objective finite concept. Who here wouldn't trade a 100 shares if we knew for sure that we could sell/buy later to an latent order arriving after we bought/sold it for a arbitrage profit?
Also, the imbalance between liquidity providers and suppliers is an issue since it promotes potential order imbalances over trading. A flat fee on both sides doesn't favor either side. A lower fee for volume is fine as long as it doesn't promote either buying or selling.
Finally, instability potentially arises in real world systems (vs theoretical systems) when discrete time system states don't exist. The digital computer jumps discrete state to discrete state no matter how fast it does it. (As has been pointed out in this thread earlier - that is what the latency delay is for. IEX simply guarantees that data is equal for all market participants. Something that has been promised by wall street for decades - wink wink, nudge nudge.)