> This too smells fishy and rigged to me. Modern databases don't need T+2 to achieve consistency.

The problem is, you have lots and lots of databases-- there's not one central ledger of every position. Settlement is effectively an atomic commitment protocol, so that if you are selling a stock you expect to receive from one firm to another, and someone else fails to deliver, you need to be able to unwind the downstream trades-- but also need all transactions to eventually become final.

The T+1, T+2, T+3 periods are convenient in human units, because sometimes it's desirable to have a human in the loop when unexpected things happen.