I'm just one guy with an opinion, but I strongly disagree that Facebook is a model example of a complex website. Facebook is blessed with a money hose, that ultimately washes away so many other problems, meanwhile provided that Facebook is not a network hardware and CDN company, I'd contest that if that's what they wind up bragging about, incidentally, then that's clearly a hint that they've gotten their core offerings wrong.
Yes, Facebook started as a PHP garbage pile, and now it's got more going on, but the surface UX of Facebook remains a garbage pile, whether it's a fast garbage pile in terms of line speed, or a photo-realistic representation of a virtual garbage pile.
If I can get consistent browsing results, productive interactions, and notice a presentable style and appearance on a React documentation website, which is a wholly-owned property of Facebook, then why isn't the flagship Facebook system provided to end-users with the same nuanced attention to detail?