I agree that we are overthinking about infrastructure. Boring stack like traditional RDMS, single server with regular backup, few bash script for deployment is fine for normal startup that targets to non-tech customer. They will serve you well at least one or two years, then you will know what should be improve. One of the big surprise is database like PostgreSQL can handle like 100tps very well with cheap hardware cost. That mean you can handle up to 86 millions transaction per day.