We can use depository receipts and similar strategies to keep trading 24/7. But the markets would probably be a lot more efficient if they were open all the time. That’s not to say that we (traders) would necessarily make much more money off them that way. We’d probably make less, actually, because it would simplify/remove the kind of arbitrage market opens/closes let us do.

The traders still sleep and stuff. The efficiency loss was time wasted that was scheduled for trading, but not used. It's no different from unexpected downtime in any other job.

I guess I question why exchanges haven't moved toward a highway or hospital model where it's open, unless it really needs to be closed. Those places have the procedures in place to operate after hours.

Or at least adopt the retail model where they get some more retail traders by operating some weeknights/weekends.

FX trading is at least 24/5, there may be venues trading on the weekend but I expect spreads are very wide. CME is open 23/5 already. EUREX has extended their trading hours into the Asia shift in the past few years, I think 20/5. Many other European and Asian derivatives exchanges have extended trading hours over the past decade.

Most equity exchanges are only open or liquid during local business hours. US markets are open longer, but generally illiquid and there are fewer protections outside of "market" hours.

I think for equities, nobody wants to be a market maker 24/7 without a sufficiently wide spread to protect them from news events such as the death of an executive. So even with 24/7 markets you'd probably see very poor conditions outside of core hours. With derivatives, there's more interest in never sleeping. The world is a very interconnected place and less hinges upon a single person's death.