These CRUD apps need complex business rules, requiring expertise in the domain and making them configurable on the application level for customer while trying to keep the app not bloated.
Scaling is not the only challenge engineers face, but somehow it's the one that is mostly praised.
They also need to respond to customer requirements, which IG never needed to do while they had no actual customers. And as soon as fun was up and IG had actual customers (spoiler alert, advertisers) what a surprise 3 devs was not enough.
They also need to quickly respond to downtime, because unlike IG if some of those CRUD apps go down in B2B world you are often losing customers actual money not just ad views
Sure. If you are full on marketer you can say "not see an ad for service" is as bad as "service does not work".
But anyway there was never a period where Instagram had a tiny 3 dev team and handled ads at the same time. 3 devs only worked back when there were no customers, no ads, no profits and no real responsibilities.
Scaling is not the only challenge engineers face, but somehow it's the one that is mostly praised.