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.
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.