I wasn't trying to suggest that HN needs to update the UI like this, just give an example of how even a relatively simple UI like the one we're using now is full of duplicate data. I then also gave the example of a messaging app because that's a case where you do want everything in sync all the time.

If you just rerender everything every time, then it's no problem to keep the whole UI in sync. But you probably don't want to render everything all the time - that's unnecessary work, and will break any stateful elements in the UI (such as form inputs that will get reset with every render). That's where the idea of React comes from: write code as if the whole UI is being rerendered every time, but internally only rerender the parts of the UI that have changed.

Now that has its own disadvantages, and I think there are similar approaches out there, but the point is that keeping UIs in sync is a surprisingly hard problem.