The whole fallacy of slopping together a bunch of open source stuff and not bothering to understand any of it is a house of cards waiting to fall down.

It's far better if you have the time and resources to develop your own software from as low a level as possible. Each layer of abstraction that you can shed is an opportunity to tailor your solution more closely to your problem and to have expertise in-house. Big tech companies know this and it's why they do a lot of stuff in-house. The key is in knowing what to develop in-house and what to punt on, and when.