It can start out as a business and then get acquired by a bigger business like Stripe, paypel, business, mercury. Either by purchasing the company whole, or by contracting to implement the feature on their systems.
I agree, that seems like the most reasonable exit strategy. From what we know of the model, it's something that could reasonably get built by Stripe into Stripe. I wouldn't be surprised if they're already working on something like that.
I worked in Ecommerce with a company that built sophisticated subscription solutions well before Shopify had a real offering. We took significant VC money, then watched Shopify build out their own solution to service the now proven market. I'd expect them or Stripe to do the same thing here; it's way more common than buying the first mover or hiring them to build it, plus YC has no interest in this sort of business model.
