I admire the work done for open access in South America: SciELO is a wonder (although only few hundreds articles are correctly marked and indexed as freely licensed) and some open archives are extremely effective, with very high OA rates.

However, I've read the article twice and I still don't get its connection to Plan S. Looking for the source of my confusion, I think I might have identified it in this sentence: «We understand that Plan S implementation may encourage a scholarly journals shift to article transaction models dependent on APC». This is a paralogism: it assumes as true what presumably it indends to show, namely that Plan S would encourage APC-funded gold open access by for-profit publishers. If the entire article is re-read substituting "Plan S" with "hybrid OA" or similar, then I understand it.

It would be interesting to hear more on what challenges the authors see in recruiting authors and funding from the areas most involved by Plan S, in directing them to SciELO and other academic-led or library-led green and gold non-profit open access (or diamond OA) such as http://oad.simmons.edu/oadwiki/OA_journals_published_by_libraries , and in preventing South American authors from migrating to more expensive OA models in the mistaken belief they might be more prestigious.

Update: in https://www.nature.com/articles/d41586-019-02857-1 I read that «Plan S has yet to demonstrate that it will also support the advancement of non-commercial open-access initiatives». I still don't understand why, but let's put it aside. I don't understand whether the criterion is "non-commercial" or "non-profit", or something else. If "non-commercial", it would be useful to define "commercial", because that's a very tricky term.