It is an excellent comment. I learnt a lot!
T+2 must prolong the agony. So new users turn up (which costs capital for their promos), deposit money (which drones clear yet), buy something (using more capital), sell it and buy something else because why not lock in those gains (using more capital) and so on.
What do brokers do in these cases? It seems like they should have a lender who guarantees capital in the even of volume spikes. It seems like a missed opportunity, I’d lend them cash for a few percent a week :)