But how would HN avoid creating its own /r/all ?
If not in the content-mix sense, then in the sense that there is one and only one homepage, and that it's everything posted to the site.
Though in truth: there are some alternate views. "Pool", "Invited", and "Best" are subsets of submissions. You can also do a blank search on Algolia and set a time window to view the best items of the past day, week, month, year, or other arbitrary interval.