I think this is a worthwhile point. My belief about Substack is that many of its initial appeals to writers were to people who were not happy at traditional pub

I think this is a worthwhile point. My belief about Substack is that many of its initial appeals to writers were to people who were not happy at traditional publications. People who had brand names, and wanted better money, prestige, or editorial control than they were getting. So, it's definitely designed to provide isolated perspectives: isolated not only in the sense of being a magazine with a single voice, but also in the sense that it will tend to attract people a little further to the margins of the cultural discourse, who attract attention and bump up against editors. However, I don't believe this is a big problem, since there are many other places to get opinion and analysis other than Substack. In fact — and I have no basis for it other than intuition — it doesn't feel like Substack's model could ever make it that big. By nature, a magazine with one writer isn't going to provide enough content to be anybody's only source, and, as you say, there aren't that many people who are going to subscribe to dozens of Substack writers. Having tens of thousands of subscribers really doesn't make too much of a dent in the media landscape over all, it's just that it's very well monetized, so it's great for this handful of writers (and Substack Incorporated of course).