Thanks for your comment, Smillew! Glad someone is bringing another perspective :-)
Here’s my rebuttal.
The first thing I’d like to say is that we writers on Medium are generally working for pennies. So $6 for bringing one new reader in is a pretty good deal for most of us. Just look at how many writers are already plastering their Medium links all over the web in the hopes of getting a few more views on their stories. They are already working for far far less than $6.
But even if you were one of the superstars who are already making boatloads of cash, I think you’d still be incentivized. First, you have a much larger reach and you might be able to bring in hundreds of new readers to Medium with as little work as an occasional tweet. Moreover, just make it 12 months that the reader has to stick with the referrer if you want and then, by default, let the writer continue to receive the 50% as long as the reader does not explicitly cancel the patronage.
With this setup, most readers will probably not move their patronage elsewhere. So a good chunk of the readers you’ve recruited will probably stay with you for a long while anyway. But the few that want to move their support elsewhere, have the option.
And if you allow readers to become patrons of more than one writer, then you wouldn’t even necessarily lose them if they want to support someone else also.