I think all of what you are describing is a symptom of letting the "co-worker" facet of a relationship dominate all other facets. In other comments I've been using the term "work friends", but I use that term loosely because the reality is that they are first and foremost "friends", it just so happens that our friendship was first kindled through work.
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.