Should someone who just missed a flight that later crashed feel lucky?

It is an interesting philosophical question, but I think the practical answer is that yes, they should feel lucky. As we don't know whether our universe is deterministic or not, we can't make the judgement call on that account. So let's ignore that possibility for the moment.

What elements can we consider when looking at whether or not the flight would've still resulted in a crash if the person hadn't missed the flight? For starters, what caused the crash? Was it an engine malfunction? Did two planes crash into each other? Did some kid with a laser pointer blind the pilots?

If it was some mechanical issue like an engine issue, that was going to happen whether or not they were on the plane. But if it was a timing issue like two planes crashing, there is more room for the possibility that they would not have crashed if the person had been on the flight.

Was the planes departure slightly delayed because they were expecting the person was going to make it to the flight? If they had showed up on time, maybe the flight could've gotten out a couple mins early. Or alternatively, if the person had shown up, but made a scene during boarding and made the plane lose its spot in the queue, the crash might've been avoided.

In any case, if I were in this situation, I would feel very lucky to have missed the flight.

It is survivor's guilt (https://en.wikipedia.org/wiki/Survivor_guilt) versus the premise of the Final Destination movies (https://en.wikipedia.org/wiki/Final_Destination).

I wouldn't take it too seriously. No one can tell you how to feel about it or if it means anything or not.

If it troubles you, talk with someone about--a mental health counselor if needed.

What do you mean by “should”?

Do you mean it would be morally wrong if they did not?

Do you mean they would have some sort of neurological or personality disorder if they did not?

Do you mean something else?

"misfortune begets fortune, and fortune begets misfortune. This goes on without end, and its depths can not be measured."

From the parable of The old man (who) lost his horse: https://en.wikipedia.org/wiki/The_old_man_lost_his_horse

"lucky" is a word we came up with to pretty much describe those circumstances.

Since luck has so much opinion wrapped up in it, it's really easy to change your viewpoint and describe the same event as not-lucky.

Or maybe they die soon anyway?

Maybe they were suicidal.