You are not understanding what MLMs are, how they work, and what makes many fail.
Let's go with the assumption that there are unlimited referrals (which I got wrong, so this is purely theoretical now). I'll try to explain my thought process:
There's a finite number of eligible people who can take part in a MLM which is why it ultimately fails, because eventually the (main) revenue stream dries up - the initiation fee. Additionally, there are no more buyers of the product since everyone are sellers, so the other revenue stream trickles to a stop.
Similar idea here - the only way to actually make money (at least more than any other similar financial instrument), is to keep on referring people in order to maintain the interest rate.
Eventually you run out of people to refer since everyone (you know) is already a Marcus customer, which leaves you earning only 4.15% which is much lower than what you can get elsewhere. This is the aspect that made me compare it to a MLM.