Tongue in cheek response, but could I suggest that as an economist you talk to a marketer before posting something like this.

The people who are going to have the highest referral conversion rates are going to be authors that are promoting themselves first. "Subscribe to me and also get all these other great things in the rest of the Medium subscription."

Consider a best selling author promoting their own writing on Medium to their own fans versus a similar best selling author promoting Medium without actually being present here. The first will be have a 10x higher conversion rate because it’s a 10x more compelling pitch.

The authors who are capable of successfully driving subscriptions are also exactly the types of authors that would also drive retention here on Medium. But they are also used to getting a rich payout because their alternative options include building and hosting the entire subscription themselves, i.e. using a tool like ConvertKit.

I have a hard time thinking random people with big audiences are even going to attempt to act as pure affiliates here (which seems to be your concern, i.e. "favours people who have huge followings") . And even if they do, the way to look at this is through the competitive lens of what other affiliate programs they have access to. They'd be better off as affiliates for vitamins and makeup. I practically guarantee it.

But mostly, I think this is best explained by a marketing lens. The referral pitch is really only going to be effective if it is given by an author who is already on the platform. And so in this way the program is great for a certain type of high level writer.

And those types of writers are great for all the writers here because they grow the pie. That high level writer gets a referral fee, but all the authors here get to compete for the remaining subscription fee of a reader who wouldn’t be here otherwise. That's a huge improvement over the status quo.