You'd only typically lose cents (if that; often enough you don't lose anything) per share. And there's no easy way to validate the price is "fair," since share prices are so fluid.
It's also a fairly victimless crime, as the article notes (although it downplays this); in practice, it seems users typically saved more money thanks to Robinhood's zero-fee trading than they lost in less optimal pricing for their trades. The problem here wasn't that payment-for-order-flow is bad — even Schwab does it for their zero-fee trading platform, and in practice it's often a good tradeoff — the problem is Robinhood lied about it.
And the fact that RH kept a far larger portion of the price improvement as their kickback. As the article mentions, the standard brokerage cut is 20% from the price improvement.
RH demanded 80% of it for themselves. Sure, their customers still got a better price than available in the lit exchange(s) but they were bilked by the 4x hidden commission.
It's also a fairly victimless crime, as the article notes (although it downplays this); in practice, it seems users typically saved more money thanks to Robinhood's zero-fee trading than they lost in less optimal pricing for their trades. The problem here wasn't that payment-for-order-flow is bad — even Schwab does it for their zero-fee trading platform, and in practice it's often a good tradeoff — the problem is Robinhood lied about it.