That's the catch. It's easy to start and experiment. But as you grow bigger you start realizing it's a bad deal so may start looking for alternatives. That's bad for Substack since why would you like your most successful users to be unhappy? A better model IMHO would be a fixed tiered price based on the number of subscribers you have, similarly to how most email providers' business model work. For most users it wouldn't be cheaper than 10%, but for the top X of users it would. Having a fixed cost you can plan in advance for is more attractive IMHO.
you say this but there are dozens of people running million+ dollar substacks with no plans to leave. this market is huge, writers really don't like to futz around with tech as much as we do