Is it really though? It very much depends on your definition of 'scaled up'. Sure, you wouldn't run a Fortune 500's payment processing through stripe's public pricing plan.

But for a $10M SaaS startup, this would come to $350k/yr (assuming some amount of non-credit-card and non-taxed payments). I would say at least 60% of that you would pay anyway to other payments processors, even doing all the software stack yourself (nothing is free in the world of finance, after all). So that leaves you with $140k p.a. for a software stack that covers billing UI, invoicing, taxes, financial reporting. It's far from obvious how you can come up with a comparable solution yourself with a budget of at most 0.5 developers and 0.5 designers that your $140k would get you.