You can say the same thing for many saas companies. Anyone can clone Twitter, Facebook, Instagram, etc. But there are network effects. I like substack because they have a free tier and easy monetization. If I were to monetize I'd probably go with them due to ease of monetization, their interface, transferability, etc. The differences between fees in competitors is probably negligible.

It's differentiated to something like Medium because I know I won't get ads, pop-ups, arbitrary paywalls, cheesy SEO content, etc.