I think the fair side of this is that you have to make tradeoffs when you design things. Scaling problems are design problems, but whether they were mistakes or not really depends on how predictable that scaling was.

Car analogies are typical, so I'll add in there.

My car can take the four of us, and we can load it up with things from the shops. I can put a bunch of heavy tins of food in there, or some DIY things, but if I put several tons of stones in the boot it'll totally fuck it up.

Is that a design problem?

Not really, it's a relatively cheap regular car, and it failed at a certain scale.

It would be a design problem if it were a flatbed truck, despite it being the same scaling that showed the problem.

Making my car resilient enough to take that weight would require tradeoffs that would either make it worse for other jobs I want it to do or at least add significantly to the cost.

This is similar in engineering software systems too, you can make it handle scaling up better, but this can require a much more complex architecture that may make it slower at smaller scales. It can make it more complicated to work with, add additional risks of failure as well.