It doesn't even have to be medical emergencies that go unnoticed. There are other declining health metrics that an independent observer might notice that a person is not aware of or is unwilling to confront. If there's no one to say "you might want to get that checked out", you probably won't get that checked out!

No mistake, almost all of us can refer to an anecdote of an elderly relative dying soon after their spouse. It can be both tragic and, in hindsight, romantic. But really, the consequences of loneliness are often and unfortunately quite practical.

Its anecdotical (though, I guess, there is plenty of data for that), but often people (especially men) don't visit a doctor early enough. It takes their partner (of friends, kids) to prompt them to get checked.

Medical emergencies might be a cherry on the cake - but let's not forget that most diseases are not instantaneous - and sooner these are cached, less harm.

I saw an analysis that demonstrated this clearly by replacing loneliness with population density. Effectively you could see the same mortality effect for people who live out in the sticks.

The article cites many measurable negative health effects aside from death, so it seems that the emotional impact does matter a lot.

This is a very valid question. I have had a few close calls where I afterwards wonder what would have happened if I had hurt myself seriously. Chances are, no one would have paid attention in days.