We had web pages with big lists and tables in the DOM 20+ years ago, they were fine. The difference is that now we use web frameworks that do work proportional to DOM size many times per second.
Call me a conspiracy theorist, but I think it's all a plane to make it harder for people to save stuff. If the content just stays there after you loaded, you could just save the page as HTML and, if there wasn't a lot of javascript shenanigans, it should save it okay. When you add this element, this doesn't work anymore. I'm pretty sure instagram, for example, does that with the intention of making it harder for people to save profiles.
