One issue the demos reveal is, it doesn't _feel_ like the web. That is, I can't hit Ctrl+F to find text on a page. I can't select text with my cursor. I can't copy the address of a hyperlink. On my phone, I can't hard press on an image and share it to others. Screen readers can't handle it. I can't press a shortcut key to make everything larger.
These all may seem pedantic, but they contribute to the feeling "this is not the real web."
This is the same problem with Java applets in the late '90s, Flash and Silverlight in the early 2000s. They are islands of richness within a web page, but those islands are, well, opaque to browsers, search engines, and virtually all web tooling.