Worth keeping

I think there is a limit on replaceability of friends, even if you are surrounded by people you like. First, a part of value in friendship comes from understanding each other. Exchanging information about each other takes time; with a new friend you have to start from the beginning. Second, if you know a person for a short time, you don't know whether their current behavior is typical, and you don't know what their bad moments look like. So you can rely on them less than on a person you have known for years.

I would still assume a difference between environment where potential friends are scarce, and where potential friends are plenty. But the cost of replacing a friend cannot become arbitrarily small.

Some reality checks:

• You are supposed to make good first impressions. That makes sense according to this model, because for a person you just met, you are most replaceable, so you have to try your hardest.
• You can be more open about your weaknesses to people who are strongly connected to you. Makes sense; the least risk of being replaced. On the other hand, sometimes you can be very open to strangers (including therapists). Also makes sense; in this case you will certainly be replaced soon, so there is nothing to lose.

If there are better replacements in general, then you will be inclined to replace things more readily.

The social analog is that in a community where friends are more replaceable—for instance, because everyone is extremely well selected to be similar on important axes—it should be harder to be close to anyone, or to feel safe and accepted

I can come up with a countervailing effect here, as well. Revealing problems is a risk: you might get help and be in a more trusting friendship, or you might be dumped. If there are lots of good replacements around, then getting dumped matters less, since you can find someone else. This predicts that people in communities that gather similar people might expose their problems more often, despite being replaced a higher fraction of the time.

Another difference between cars and friends is that you're going to get equally good use out of your car regardless of how you feel about it, but you're friendship is going to be different if you can credibly signal that you won't replace it (taking the selfish-rational-individual model to the extreme, you probably want to signal that you'd replace it if the friend started treating you worse, but that you wouldn't leave it just because your friend revealed problems). In a close community, that signal might get worse if you repeatedly replace friends, which predicts that you'd be less likely to replace friends in closer communities.

No empirical evidence of any of this.

It's an interesting tradeoff, but it doesn't come up much, for me. I think, in most relevant domains, people aren't actually good at hiding their problems. Humans seem too complex, too expressive, too transparent. We were not adapted to effectively wielding privacy. We cannot fake important skills or insights that we do not have: We don't know what we don't know, we don't know the tells.

In order to present a convincing picture of a human being, clear enough for anyone to trust you with anything, the only way most people can do that is by telling the truth.

in a community where friends are more replaceable

Wouldn't this help people open up? If you're worried about losing people you might try to be more careful. If the replacement cost of something is lower, then you can afford to take more risks.

A related question is when you should let people know where they stand with you. Prima facie, it seems good to make sure people know when they are safe. But that means it also being clearer when a person is not safe, which has downsides.

An interesting question.

Even if you are not specific about where people stand with you, they have the evidence of your past actions. So whenever you decide whether to stick with a friend or give up will provide evidence to your other acquaintances as to what they can expect from you.

If one is perceived as being too quick to ditch friends, it probably decreases availability of replacement friends. On the other hand, someone who is extremely loyal is likely to have greater availability of friends (up to a limit!) but also less need for new friends.

This surplus may give leverage for things one cares about - one might say "I'll stand by my friends but I do expect they turn up when they say they'll turn up". Someone who is less loyal may not be able to be so picky.

Deserving of thought and study, but risks going down the evpsych path of cute but useless "just so stories".

Attachment to friends (and for many, to cars) is more complex than this - you have to include signaling (and self-image-signaling), and way more options than "invest or replace".

Heart / Sad / Nod React

*wiggles fingers* <3