No. The issue is that you have no central ledger of ownership of stock, but instead many firms' individual ledgers. You also have various kinds of risk of fraud, etc.
T+2 is an atomic commitment algorithm that leaves enough time for humans to make phone calls and discuss the implications when there's going to be a failure to deliver or another anomaly from fraud or globally inconsistent state.
It's far more centralised than you think, dematerialization isn't 100% complete but Cede and Co. hold the ledger of who owns what - at least to the extent that is possible.
T+2 is an atomic commitment algorithm that leaves enough time for humans to make phone calls and discuss the implications when there's going to be a failure to deliver or another anomaly from fraud or globally inconsistent state.