It’s doing well because more and more writers are joining and playing the follow4follow (or rather, subscribe4subscribe) game :D

Money wise, Substack is hit or miss. You can make much more than Medium, but few writers can convince readers to pay $5 for a dish when you can have access to the all you can eat buffet for the same price.

I wrote about Substack recently

https://medium.com/the-pub/how-to-increase-the-number-of-your-paid-subscribers-on-substack-fea18f4284a2

And you’re welcome to subscribe to the genius tier. It comes with benefits

https://medium.com/the-pub/supercharge-your-substack-subscribing-page-with-this-example-6cf895bdeab5