Marcia McNutt, distinguished scientist with extensive publishing experience and now Chief Executive of the National Academy of Sciences (US) offers an opinion piece on Plan S. The link has been widely circulated, but if you haven't paused to read the piece, please make the time.
Opinion: “Plan S” falls short for society publishers—and for the researchers they serve"The proposal known as Plan S has the admirable aim of achieving full OA across a wide swath of journal publications. But the path currently suggested has serious drawbacks that could jeopardize nonprofit science societies.
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https://www.pnas.org/content/early/2019/01/24/1900359116