Payment for order flow doesn’t mean you’re losing good order execution.
I'll be honest that I don't have the full picture, but happy to learn something new if it's a different story.
Because of this, it's legally forbidden for them to mess with your prices in such a way that you lose anything.
I think people see that these firms make money from selling order flow and as a result assume that they must be losing money somehow. That isn't the case. Some firms (e.g. HFT) are simply willing to pay a little extra for the counterparty to their trade to be a dumb joe instead of another HFT firm. This can lead to both you and the broker winning.