You can get a long way with a monolithic architecture and a reasonably good database that you don't abuse nowadays (40-vcore servers, hundreds of gigs of RAM, terabytes of fast SSD). And in the face of getting a product out as quick as possible, not making hugely awful decisions in how to store and access your data is key. 10% more time thinking about these problems saves you several orders of magnitude down the line fixing and solving scaling problems.
If you really scale, you're going to rewrite most of it eventually anyway, but having the data stored in a sane manner makes all that easier anyhow.