The logic is simple: an individual writer is always relatively niche. Niche means small audience. Small audience means you have to charge more per person to cover your costs.
Maybe Substack will go the Medium way and try to aggregate, or encourage its writers to aggregate. If so, then it could succeed, but it'll succeed by becoming a newspaper. The problem is that the NYT, Guardian, Mail etc. have a lot of experience at doing aggregation well. (Each day has an appropriate mix of home, foreign, finance, and sports. The day's editorial chimes with what the front page journalists wrote about. Long run investigations are funded in the hope of big payoffs. Et cetera.) Medium hasn't really broken through, I think Substack would struggle.
An alternative is the status quo: Substack is a perfectly viable blogging tool, where aspiring journalists and writers like me [1] give away content for free, in the hope of breaking through to the next level, and where a few people make a good living. But if so Substack will fit into the existing landscape, rather than disrupting it.