Stripe has an extensive ecosystem of services that integrates with it, literally hundreds (https://stripe.com/works-with).
The services I need to run the back-office operations of my business including bookkeeping, invoicing, receipts, business metrics, customer support etc. all use Stripe data and APIs. Writing our own integrations, or worse, trying to poorly replicate some third party service, would be money poorly spent. It would take a very large transaction volume for it to make any sense to spend engineering time here, and arguably the end result will be qualitatively worse than just going with the market leader. Even if that calculation does come out positive, you still have to consider whether the engineering time wouldn't provide an even higher ROI by being invested into your core product instead.
Integrating with a merchant payment gateway isn't rocket science. A few weeks is easily worth the cost (it doesn't take that long, either, most have decent enough APIs or SDKs that may not be stripe quality but still are simple to integrate with).
It doesn't take a large transaction volume to justify the engineering cost at all. We're talking entire percentages of revenue. The ROI justification is going to be a hard one to beat in this case.
The point was that there are N other services you use when running a subscription service, that all integrate out of the box with Stripe and not at all with the cheaper alternative.
I agree with debaserab2. We use both PayPal Merchant Processing and Stripe. PayPal processes for us at 2.2% + $0.30. We use Stripe for the smaller charges where the $0.30 is prohibitive. There are many other options out there too. For any business to stay competitive it's important to keep costs down. Merchant fees are an easy one, do some research. For small businesses, the integration work isn't that hard. I suppose below a certain threshold it just isn't worth worrying about, but I would be wary of being too locked in with stripe or any other business for that matter.
The services I need to run the back-office operations of my business including bookkeeping, invoicing, receipts, business metrics, customer support etc. all use Stripe data and APIs. Writing our own integrations, or worse, trying to poorly replicate some third party service, would be money poorly spent. It would take a very large transaction volume for it to make any sense to spend engineering time here, and arguably the end result will be qualitatively worse than just going with the market leader. Even if that calculation does come out positive, you still have to consider whether the engineering time wouldn't provide an even higher ROI by being invested into your core product instead.