I'm not GP but you are missing the most important fact: a bit over 10% of Robinhood's base revenue was PFOF. Of that more than 10%, the top 3 (~75% of the total) paying them were Citadel, Susquehanna and Wolverine.

Losing PFOF from Citadel alone means kissing away more than 5% of their _revenue_. With essentially no cost associated with PFOF, these payments account for a large portion of their profit.