I think retail trades are impacted more than people think. If anything, it seems like there's even less oversight/regulation of retail orders than institutional orders. Just look at how much money Citadel is making after buying up all the retail order flow.
Internalization and PFOF is a different issue. If someone is routing to IEX, I would assume they also (or previously) route to other exchanges. Cutting internalization/PFOF out of the loop.