I'd say IEX has done remarkably well - it's not likely to displace NASDAQ or NYSE but it has solidified its place as the #3 US exchange by any reasonable measure. If TXSE achieves comparable market share I'd call that a wild success.
You're not wrong to say that most participants don't care about what IEX offers, but enough do to make a meaningful dent in trading volume.
> #3 US exchange by any reasonable measure
According to their stats, they are usually around 3% of the market:
Cboe miax and memx are all doing better than IEX
It’s definitely not 3rd place. That’s cboe and it’s not close. It’s hanging around with ~2% of the trade volume.
https://www.cboe.com/us/equities/market_statistics/
I don’t know if that is “remarkably well” but it certainly isn’t some market paradigm shift.
If you were to tell me in 10 years the Texas exchange would have 2% of the market I’d believe you. But I’d still not be terribly impressed.