You probably mis-understand what I mean by "dumb" and why I put it in quotes. Things that are "dumb" are intentionally so; they are built on stacks using technologies and approaches that have low cognitive load. It almost feels boring because of how easy it is to build on the stack and extend the stack without shooting yourself in the foot. A very practical example is something like Google Cloud Run vs Google Cloud Functions with Cloud Run being more flexible, easier to deploy, easier to work with, and easier to reason about. It's stupid simple and rather boring.
But besides the fact, dumb things are generally easier to unfuck than complicated things. Complicated things with many dependencies built on complicated stacks are much harder to untangle and refactor.
Everyone else failed because their backend systems were overly complicated.
I don't see how there is a lesson to learn from it unless we make a ton of assumptions about (A), absolutely none of which have anything to do with angular or react.
The last paragraph definitely makes sense, though I'm currently in the unenviable position of un-fucking a startup because the initial developers chose a setup that was far too dumb. It's literally going to take at least as long to fix as it took to initially build out. Ce la vie I guess.