Huh?
SQLite is designed for application databases.
Postgres is designed for client-server.
It's not about "power" (what does that even mean?), it's about totally different design choices that are intended for different purposes. It's an architecture question, not a "power" or "visitors" question.
I'm glad we agree that "just use postgres" is horrible advice.
Would you except "Just use postgres for client-server" as a reasonable change?
We run plenty of money making SaaS on sqlite without issues. And have been for over a decade. By power I meant all the complex features postgres has. But yes, it's an architecture question; my point being, most people pick many bulldozers while they need a 1 shovel.