Funnily enough prior to this startup I had worked at a rainforest-themed big tech co where we ran all kinds of stuff on MySQL without issue, at scales that dwarfed what this startup was up to by 3-4 orders of magnitude.

I feel like that's kind of the other arm of this whole argument: on the one hand, you ain't gonna need that "scalable" thing. On the other hand, the "unscalable" thing scales waaaaaay higher than you are led to believe.

A single primary instance with a few read-only mirrors gets you a reaaaaaaally long way before you have to seriously think about doing something else.