I think you misunderstood my point, all the examples I gave such as separate mini markets, stealth exchange and clearing pools are all elements that existed in past markets that inevitably all failed.
The idea is if you allow people the freedom to choose from these options over time such options will fail - that is a certainty. The reason for failure centers around the fact that as they reduce the overall efficiency (true price) of the market, this results in a less profitable market.
For example dark-pools were invented to reduce the undesired market impact that large players may create when trying to offload large orders in a short amount of time (good idea keep the market steady) - From a technical pov its working fine today, most of the major IBs provide such services, and it is true that such services skew the true value of stocks, however as more and more people begin to utilize them, the value of such dark-pools and their advantages begin to dissipate, same can be said for the mini-emarkets.
When someone says let the markets be free, they're not talking about an "everyone is equal" paradigm but rather "let them do what they will" and let the outcomes decide. The markets are ruthless, most people are culled out of the competition before they can even blink, and unlike most other national services such as public schools and hospitals - they are not designed nor willing to cater to the lowest common denominator (be that due to intelligence, server speed or lack of optic fiber), in short they are not fair, they were never intended to be fair, making them fair will make the unprofitable - all any one government can do is regulate certain use-cases that make participating in said market unpalatable, such as insider trading/front-running.