The basic problem is when some piece of state changes, all the UI that depends on that state needs to be updated. The simple solution presented in the link is to write update functions that do the correct update for everything, but as the dependency graph becomes large and keeps changing during development, these becomes very hard to maintain or even check for correctness. Also the amount of code grows with the number of possible updates.

Reactive view libraries basically generate the updates for you (either from VDOM diffing, or observables/dependency tracking). This removes the entire problem of incorrect update functions and the code size for updates is now constant (just the size of the library).

But what if your dependency graph never becomes large (HN, Craiglist,...)?

I believe a lot of web applications can go without any reactive framework as using one is a slippery slope. You start with React and 80% of your code is replacing browser features. Imperative may not be as elegant, but it simpler when you don't need that much extra interactivity.