Except with SIPC itself being defunct. You hopefully still have priority over GameStop holders that didn't cash out yet and have busted trades from a bankrupt clearinghouse and priority over other creditors. No SIPC to cover your legal costs pursuing your claims. I think margin accounts may be behind fully settled cash accounts in line.
People have speculated that this is an area of application for distributed ledger technology, but the Australian Stock Exchange has been trying to roll out such a system and it has been slow: https://www.fintechfutures.com/2020/07/australian-securities...
Trading in securities with daily settlement, while movement of money is not operating on the same frequency, means that there is a requirement for someone to take that risk.
That's the systemic risk, not the trading itself. If the execution, settlement, and clearing of a trade were simultaneous and instantaneous, then there's no risk to the market itself.