I treat work friends as a sort of "probationary friend". I'm happy to meet up and do stuff outside of work, but I'm a bit more guarded than I would be with someone I don't work with. I might hesitate to introduce them to some of my more "out there" friends (for whatever reason I seem to attract a lot of very weird people). once we no longer work together, they can just be a normal friend.
I would absolutely share this with my "work friends" and even my current boss. When one of my previous bosses shared with me that he was having trouble at home with his spouse, it wasn't like he suddenly became a bad boss. If anything, it helped our work because it gave me the opportunity to be more understanding when he seemed more worn out or needed to leave early for the day to go spend time with his family. If he had told me he was struggling with drinking, then I would know that maybe we should go play sports rather than to our weekly happy hour. We build each other up, both inside and outside of work, because that's what friends do, and it has the nice side effect of making our professional careers better, too.
Perhaps I just have had a lucky stream of working on unicorn teams, but I would hate to work at a company where I couldn't share details of my life with the people that I spend 40-60+ hours a week with. Obviously every workplace and relationship is unique and you aren't going to share everything with everybody (I'd share the above details with my current boss but by no means with all of my bosses), but I don't understand this attitude that you shouldn't share anything with anyone that happens to have a common employer.
To me, work is just another facet of our relationship alongside the many other facets, no different than if we had met through volunteering or went to the same gym. I don't share everything with every coworker, just like I don't share everything with every other person that visits my gym. But I also don't shy away from sharing things with someone just because they are my coworker, just like I wouldn't shy away from sharing things with someone just because they also use the same weight set that I do.
also, I would say work is quite a bit different than a gym. it's a lot easier to find a new gym than a new job, and it's much less disruptive to your life if you have to do so.
My boss and I have a good enough relationship at work where I'm 1000% confident this wouldn't happen, and if said boss did "perceive" some kind of drop in productivity, their response would be to help me with that rather than turn it into a negative. Indeed, during COVID when I expressed to my boss that I was having a tough time, his immediate response was to ask how he could help and to offer me a couple days off to recharge.
And the reason we have that relationship is specifically because we treat each other as friends that also happen to have a working relationship, rather than letting the work relationship dominate all else.
Not everyone will have that kind of relationship with their boss, but I believe it is something everyone should aspire to rather than immediately rejecting it as even a possibility.
Some people will dislike you; most people will be neutral. But it's all wetware and basically instant
I've met plenty of good friends at work, but I'm very happy my social network includes mostly people doing different things with their life. It means there is much more breadth of perspective, and you realize very easily that work isn't everything.
It sounds like you also might not have had much experience in the current "everyone is friends!" startup office culture. There's a pretty strong pressure to socialize all of the time, during lunch and after hours, with everyone. The people get laid off and they are erased from that social circle, typically people won't even communicate with them anymore. It's repulsive.
For me, being co-workers is just one small facet of our friendship. We do talk about work sometimes, just like sometimes we talk about sports teams we have in common or other hobbies. But we don't only talk about work. I don't feel that my perspective is limited at all because my co-workers have introduced me to their spouses, who might be in totally different industries, or to new hobbies such as surfing or soccer, or brought me along on trips to new places or encouraged me to volunteer. All of these things have expanded my perspective greatly, and if you trace them back far enough, all of them are originating from my co-workers.
In your startup example, I have been on teams where this culture was the case, and certainly with some people I have completely lost touch after they/I left the team. But for others, even though we worked together work was not the dominating reason that we were friends. When I left the team we lost that one facet of our relationship, but our friendship still survives because we have many other facets that facilitate it.
So I think the problem isn't when someone's social circle aligns strictly with work. I think that's fine. I think the problem you are describing is when it turns into a "work-dominant social circle" rather than "a social circle that just happens to have the same employer in common".
Furthermore, I think the "anti work friend" attitude is counterproductive to this. People are saying "don't spend your time building friendships with co-workers because if you leave work you will lose your friend". But if you carry this attitude and don't build a friendship that is strong outside of work, then of course your perspective is going to be limited to work, and of course you will lose your friendship with that person once your work ends. It's a self fulfilling prophecy.
Outside of work, my "work friends" and I go out to eat, go play sports together, hang out at each other's houses, etc. We talk about work sometimes because it's obviously a common interest, but our friendship doesn't revolve around it, and if someone severs their relationship with the employer that we have in common, it does not mean our friendship also severs.
tl;dr; relationships outside of work are important and that's not sad to me because it doesn't prevent anyone from also having friends inside of work.
It really saddens me whenever I see this (apparently common) sentiment on HN. Through every job I've had, I've met some of my very best life-long friends. My wedding party is over half people that I met at work. A couple of my former bosses are people I see every couple of months (pre-covid) and are invited to my wedding. Even one of my past clients is someone that I keep in touch with regularly (for nothing related to work).
Not everyone at work is going to become your best friend, but if you're automatically assuming that anyone you meet through your job is completely "off limits" for friendship, you're really hurting yourself. I can't imagine working at a place like that.