Nobody said that. We just said "if your side IS js heavy, then rendering on the server would be nice".
Not all sites need a lot of JS. But your tweeter clone is going to be very boring if you have to hit F5 every 30 seconds and polling doesn't really scale.
My point is that many sites don't really need to be as js heavy as they end up, and people end up solving the wrong problem (how to render all this js server side rather than how to reduce all this js in the first place)
I'm not saying don't use JavaScript, I'm saying don't abuse it and then try to dig yourself out of that hole with server rendering. Use the tools you have to the fullest extent that do the job perfectly well before reaching for other ones.
Why don't you think polling scales? I would argue the exact opposite that polling scales great. To a public api can you cache to your hearts delight. To a private api you can shard away.
Also a good point. Polling is also a very straightforward and reliable system, and can be very efficiently with both caching and returning empty responses when nothing has changed, etc
Not all sites need a lot of JS. But your tweeter clone is going to be very boring if you have to hit F5 every 30 seconds and polling doesn't really scale.