Sigh. I'm truly old. That's what etiquette is for, so you have a framework to know that context.

Well, alternatively, we can use our brains and think about questions like "if I am late to this event, will other people be waiting for me/disturbed?" to determine how serious a violation of etiquette it would be to be late.

Works almost equally well. Etiquette is nothing but a way to codify the answers, a shared agreement on the rules. (I say "almost" because like any set of rules, it makes it harder to get judgment calls wrong.)

I didn't mean to imply it's a magic cure-all, just that it's a pre-shared context that makes it easy to make those decisions.