There are many reasons but most importantly, delayed settlement serves as fraud deterrent and error/exception handling. Should be T+1 same as options. Think of it as a database, do you want instantaneous non-reversible commits by default when transactions number in the billions with known error rate? Or perhaps have a reasonable buffer for safety?

For cash, pretty much every country has a functioning real time gross settlement system that manages to do essentially instantaneous non-reversible commits by default and carries all the large payments and the netting settlements for all kinds of smaller bundled payments. Yes, there are risks, errors and fraud - but if that's manageable for very, very large amounts of cash, why wouldn't it be for stock?

Most countries do not settle in real-time, or even close to it.

Even ones moving towards it are still allowing 5 - 15 minute windows on settlement.

I would go even further. If I am building a next gen electronic wallet (which happens to be a side project of mine) I’d build in a full week of escrow-like mechanism by default.

Why are reasonable buffers done in days? Why should fraud prevention be done by lag, rather than proper IAM?

And non-malicious errors are reversible.

T+0 implies same day not instant. I think you can enable all of those features with hours of delay instead of days.

I can maybe see T-0. But I am pretty sure Vlad is talking about near real-time. Even with T-0, most of the trading is done in the last ten minutes at 3:50 EST (due to vwap). I don’t see the benefit in settling T-0 outweighing the value of safety net provided by extra time overnight.