Maybe for the clients as well as they're getting better prices. Market makers lose with toxic flow (on balance), retail is not toxic and never will be so bid-offer prices ONLY for retail can be tighter/better than for large potentially toxic customers/flow. Selling flow to Citadel is therefore good for the small clients. Go back to 1/8th and 1/4 and the old floor for fills versus what retail has now.
re: rare phenomena in life. +1 exactly. you understand the reality. it is a triple win win. "It is a win win situation for the two companies. Those "few pennies" add up to large profits." + the retail trader is a big winner.