How can you tell they will definitely cause an accident?

The same reason you're not supposed to drive or operate heavy machinery on a wide variety of medications, including legal and over the counter ones.

Sure, but op is talking about someone who uses controlled substances outside of work. If they’re not under the influence of a substance at work, then it’s not the same situation you are talking about.

The effects of almost any narcotic last for 72 hours at least. Not as intense, for sure, but there are still symptoms.

The effects can last a lifetime but saying any narcotic will last for at least 72 hours doesn't take into consideration dose, weight, history isn't even closely true.

Did you know eating a poppyseed bagel will cause a positive for heroin for upto 60 hours afterwards while doing a small amount of heroin will not show up on that same test.

Should people who drink get fired because they might drink on a Sunday and come back hungover?

Depends on what they are responsible for.

Airline pilots, I believe, have a no-drink-within-X-hours rule. I'm not sure whether X is 12, 24, or more. So even one drink the night before a morning flight would be prohibited, even though logically a single drink with dinner the night before should be a non-issue.

“8 hours from bottle to throttle”

ok, so 8 hours from vaping in the evening should also not impact your next day

I believe the aviation saying is "8 hours from toke to yoke"

If they are putting other people's lives at risk, maybe!

What kind of person is willingly going to significantly increase the loss of life or limb?

Not sure what you’re talking about here. If someone is falling down drunk, k, get them out of the machine shop.

Because they showed up on depressants that are known to inhibit your ability to operate heavy machinery?

How can you tell they are on depressants? Did you drug test them?

I can see where you are going with this, and I am aware of the problems with drug tests and largely agree they are over-applied. But I don't know of an alternative to these imprecise tests in many cases.