Honestly ALL diagrams are a dead end, any system where there is no single source of truth will be a big hassle to maintain and therefore will not be updated which makes the whole thing useless.
The only which I can see working as it has already proven itself for certain tasks is something like the blueprints from the unreal engine but where each node represents something big enough to warrant its own node without any no low level details like variables, conditionals, loops etc. Otherwise you end up with literally spaghetti code.
This is an excellent point. You need to pull all the information about a system in a single place so that then you can choose what level of abstraction or deep dive into the details you need.
Projects like Multiplayer.app are in their early days, but I can see the potential of focusing on concentrating this info and automating the maintenance of docs and diagrams.
The only which I can see working as it has already proven itself for certain tasks is something like the blueprints from the unreal engine but where each node represents something big enough to warrant its own node without any no low level details like variables, conditionals, loops etc. Otherwise you end up with literally spaghetti code.