This is why there's an endless cycle of shitty SaaS with slow APIs and high downtime. People keep thinking that scale is something you can just add later.

What's a more reasonable general approach then?

Let's say you're a team of 1-3 technical people building something as an MVP, but don't necessarily want to throw everything away and rewrite or re-architect if it gets traction.

What are your day 1 decisions that let you scale later without over-engineering early?

I'm not disagreeing with you btw. I genuinely don't know a "right" answer here.

I'd argue on the contrary that it's the last decades' over-engineering bender that's coming home to roost. Now too many things have too many moving parts to keep stable.