>>zie+RI
i mean, sort of? There is some subtly lost in this oft-repeated advice. i've worked at 3 companies now that were initially based on a single RDBMS but have outgrown the scale of what is reasonable to serve off that architecture. They are consumer scale (10s of mill) users, but not hyperscale (IMHO 100m+). The amount of engineering cost to migrate a complicated growing company/product off a mono-db architecture is
astounding. Conservatively i'm talking 10+ dev years of effort, at each company. Easily 10s of millions of $$$, maybe 100m+. None of them are "finished". It's really really time consuming and hard, once you have 100s of tables, 100s of thousands of lines of code, dozens of teams, etc.
I'm all about avoiding premature optimization, and its fine to start with a classic postgres. But please don't cling to that - if you see MVP success and you actually have a reasonable chance of getting to >1mill users (ie, a successful B2C product) please please dont wait to refactor your datastore to a more scalable solution. You will pay dearly if you wait too long. Absolutist advice serves noone well here - it really does depend on what your goals are as a company.