Simply make it so that when JS isn't being used by the link acts normally.

You have two option with the web. Either you put your logic in the backend or in the frontend.

This addition is obliviously to reduce the load on the server. So either you add some logic on the frontend to make the user experience good or you do like they do right now and have a bad user experience.

No JS needed. You can do this with CSS using the target selector and display:none. But that would require loading all comments up front which is maybe what they're trying to avoid?

Not really, you could just load the same page with something like ?comments=all from the server.

That seems better actually, considering "more" doesn't really make sense when the things you've already gotten may have shifted in the mean time.