But I'm not sure they can be eliminated entirely. At least sidenotes make use of empty space on wide screens.
This is true for both footnote and side note, but for footnotes I can give it a glance to see if I want to read it thoroughly after reading the whole page, whereas for side notes, they are so easy and tempting to be seen. I feel the point of notes is that they do not interrupt the main flow but side notes put them on almost equal footing in terms of attracting attention as the main content.
Besides, footnote/side notes should be kept minimal, otherwise the note becomes the main content. I find articles with side notes often overuse them. Yeah, if they are kept minimal I guess both approaches are fine.
There are some authors whose footnotes (even sometimes indices) I read before the main content, because in the past they've hidden all the best bits there.
I very rarely use footnotes except for citations, unless it's a precision that would be useless to all but the most erudite readers.
When I read articles with side notes, I often find myself constantly checking the side notes and then regret doing it because the info is unnecessary. I prefer the author to carefully think through and choose what info to present instead of just dumping info to the side notes.