Yes, it is arbitrary, and not a fixed number. And yes, a lot of users could improve.

Forming closed groups is not how you build a big community. If we want steem to grow, it's important that a lot of people look outside of their group. Especially the ones with a bigger stake - if they group up with other big stakes, there won't be a community except these groups.
And past experience shows that very clearly. We had thousands of people join over time, who left again because they didn't feel like valued members of the community, because of group behaviour. Retention is the number one problem, and the only way to improve that is to break the circles.

I'm absolutely aware that it's not a realistic expectation that everyone scouts the blockchain all time. I don't do that myself. That's why I started a curation project that makes it easy and still profitable to distribute to the wide userbase for myself and every other stakeholder. They can still keep some for supporting their group directly. But as someone said earlier today, at some size you have to stop being a boat and become the tide, or there won't be enough water for all the boats.