In my experience, this is not much of a problem. Server-side rendering is not that expensive, and can easily serve high volumes of users. After all, it has been used for decades, and is in continued use by most major sites. There's no significant performance difference between rendering HTML and rendering JSON.

Furthermore, there are easy ways to mitigate the cost of rendering on the server side. For example, on a site like reddit, most of the traffic is probably non-logged-in readers hitting the front page, so... cache the front page. Even for logged-in users, the front page listing only could be cached as rendered, since none of it really needs to be live. If liveness was desirable for only certain elements (e.g. voting buttons), it is possible to sprinkle on client-rendered elements.

but... then it would be the thing it's trying not to be.