What are the mechanics of that?
Let's say I buy a share of F on Monday, my brokerage routes it to Citadel, because PFOF.
On Tuesday, I expect to get a share of F delivered at close of business, because T + 1.
If Citadel doesn't deliver on Tuesday, what happens?
Are you suggesting they would continue to not deliver the share I purchased for several days, by saying oh yeah, we'll get toast0 his shares tomorrow? That would be pretty upsetting, and I imagine I'd call my brokerage and ask them why they're dealing with Citadel if they never deliver shares on time.