You know what also uses the streaming parser? Standard server-rendered pages. The idea that SPAs are faster because “they just download the content” is rarely true, HTML compresses well and the content will take the same space in either. The real bottlenecks are in re-executing huge amounts of JS per page (don’t have huge amounts of JS?) and page transitions (being worked on).
It's also pointless for users who aren't the lowest common denominator, since opening links in new tabs also defeats the whole premise. That's how I'm browsing HN right now: middle-clicking several links on the front page, so I don't lose my place, opening the links and then comment pages in new tabs.