I think the good question would be: say you are Google and accidentally lost your code base (I heard it's a java monolith with around 5000 major intricate dependencies, something unsplittable). How would the Google eng team rewrite Google if they had to? I'm kind of 100% sure they would not go monolithic. They more likely would have services built upon a platform, maybe not nano services, but still services that can be handled by a small team with well defined responsibilities.

The same with Facebook and php or stackoverflow and .NET: a successful start-up doesn't prove it's tech stack is the best, it just proves it can work. Someone who left to build a new ambitious competitor is more likely to choose a better tech stack, e.g. Python for Quora.