I strugle to interpret the grandparent's phrase "You want to be as exclusive as possible with the other person, so that you both have as much of other's attention as possible" in a way that doesn't support socially isolating your partner, which i would consider unhealthy as an ideal, and abusive in practise. All relationships take time and effort, whether that's friends or otherwise. If 100% of your attention (which is the max possible) is focused on your partner, it implies that 0% of your energy is focused on any other friendships/social contact. That is unhealthy. It should not be held as an ideal.
But to be very clear, you can still be in a healthy exclusive romantic healthy monogomous relationship, while still having other friends & acquaintances. Having other platonic friends is not the same as being polygamous/polyamorous/etc.
It is. If you go into a chatroom with 10 active people its easier to form bonds with any particular person in the group than if you have 400 people. A relationship on HN or Facebook is extremely shallow because it is not exclusive. If you were to restrict yourself to bookmarking to 1-5 users and only responding to their comments then you would easily form a relationship with those people. Lots of people put their email address in their profiles so they can have exclusive communication between two parties.
Maybe the social media example is too tech focused. A relationship between parent and child is also exclusive but the exclusivity shrinks as the family size grows. There are extreme families that have 10 children and as a result each child spends very little time with their parents. Instead, the older brothers and sisters take care of a lot of parenting. Children form stronger bonds with their siblings than with their parents.
>Even in the context of a traditional monogamous romantic relationships that sounds incredibly unhealthy.
That sounds to me more like your own preference is to be on a polygamous relationship with several people. There is nothing wrong with that but calling other people's relationships "incredibly unhealthy" is quite rude. It's easier to have a deep relationship with one wife than a series of potential girlfriends who you meet for one date and then never again. Of course you might find the right one and then never marry but from an exclusivity standpoint it is equivalent to a marriage.
Maybe you are reading too much into "have as much of other's attention as possible". Exclusivity is not about how much time you spend with a person. It's about how much you are restricting yourself to a small selection of people. Exclusivity increases the maximum possible time that can be spent on a relationship but it doesn't mean that you actually spend every waking hour with your partner or friend.