Well you know what happens when we assume. You make a front page headline out of you and me.
Later they began a small refactor which easily handled the loads they were "assuming" could not be handled in the way that the refactor handled, and it was wildly successful and the code was much simpler to work on.
To developers: don't over engineer. Most languages/frameworks/libraries can handle scale beyond what you'll ever get in your initial implementation. No, you entire website does NOT need to be asynchronous. It is very possible to have too many background jobs. I know this because I've seen the horror. I've also written an entire jobless/synchronous platform that serves millions of users without issue. If you run into scaling issues, that is a good problem to have. Tackle it as it happens.
Bottom line is focus on secure, quality code above all else. Don't make assumptions.
It always hurts to build something that “won’t scale” because it was framed as a negative.
Realizing that something “scales” if it meets your current needs is pretty important.
Framing scale in terms of how many people can work on it, how fast they can work on it, and how well it meets needs is often a better way of considering the “scale” of what your building.
As you said, when request per second becomes a limiting factor you can adjust your scales but doing it from start rarely makes sense (largely because req / sec already scales pretty well)
Because we don’t trust that we can keep the wheels on.
Over time the memory fades, and the confidence improves, and we get more comfortable with things being okay instead of unassailable. But it can be a rough road until then.