Earlier quoted context omitted.
Does it have to be "sustainable"? It wouldn't work for Google, but it works for Gumroad (and, like they said, most open source projects). Gumroad is a pretty simple site (and I don't mean that as an insult). They don't deal with huge customers, sales processes, large data, complicated tech, etc. Most "hard" things can be outsourced to a SaaS tool. So why overcomplicate things?
Premature optimization is the root of all evil
Great code is both simple and has the ability to scale if needed in the future.