I think what you're trying to ask is that if clearinghouse somehow knew that a given buy order would be good (ie. was paid with settled cash), then this whole fiasco wouldn't have occurred. That's true, and that's essentially what T+0 settlement is (ie. you have to come up with all the money on the day of trade).
And for that matter, how can bankruptcy even matter? These are all funds that live in a separate bucket from the rest of operations. Even in a bankruptcy, they wouldn’t disappear (like your coat at a dry cleaner’s.
[1] Recall the comment about how you’re implicitly supposing a “credible lockup of funds” primitive, which has a strange structure, on closer examination: https://news.ycombinator.com/item?id=27694540
1. the collateral is good because it's transferred/wired on the day of purchase.
2. as mentioned in my previous comment, that could theoretically be done today (ie. "trust me, I can definitely come up with the money on the day of settlement"), but it's not done for various reasons.
>And for that matter, how can bankruptcy even matter? These are all funds that live in a separate bucket from the rest of operations. Even in a bankruptcy, they wouldn’t disappear (like your coat at a dry cleaner’s.
but in the scenario mentioned above, how are you going to recover the funds? specifically, from where are you going to get the money to make the seller whole?
1. Clearinghouse member A buys 1000 shares at $200 each, costing a total of $200k. On the day of trade (not settlement) they put up $20k in collateral.
2. Clearinghouse member B buys 2000 shares at $200 each, costing a total of $400k. On the day of trade (not settlement) they put up $40k in collateral
3. Something bad happens. The price of the stock drops to $150, and clearinghouse member A goes bankrupt. They're supposed to pay $200k for the shares they bought, but they can't and those shares are now worth $150k, so they owe the clearinghouse $50k. They only put up $20k in collateral so there's a $30k shortfall.
4. The clearinghouse somehow socializes the losses, presumably using some of the collateral from member B to make up the deficit. Now member B is short $30k.
For a small amount they might be able to cover it out of pocket, but if it was sufficiently large they won't be able to. In that case the net result is that the customer had their funds seized (because their funds were used for the clearinghouse collateral) but their brokerage can't pay for the stock. Forcing the brokerage to use their own funds prevents this problem. The brokerage and their creditors might still lose money, but their customers shouldn't be affected.