My understanding of the situation includes the following: Elsevier has a new CEO. Elsevier has been reporting for several years now that revenue from services has been one of the fastest growing parts of the business, so much so that the company now calls itself an information and analytics company, not a publisher (1). Elsevier, though slow initially, is now fully behind open access. 9/10 of the journals launched last year were open access (2). Elsevier is pursuing a number of what the industry calls "transformative agreements" with libraries, consortia, and whole countries which involve full access to all Elsevier content and built in open access publishing for everyone covered under the arrangement (2). This specific issue was about one way of structuring such an agreement to reduce the financial burden on MIT while still ensuring all their content was published open access and was even designed to make it easier for librarians to keep a collection of the intellectual output of their institution by automatically pushing manuscripts into the institutional repository, which is something librarians have been asking for for a long time (3).
So given all this, the only way I can answer your question about what's stopping change is to say that nothing is stopping it. It's happening & has been happening for years. I am tempted to ask, looking at some of the comments in the parent thread, what's stopping change in people's perceptions of Elsevier? I don't just mean that rhetorically. I really would be interested in understanding why people have the views they do and how they're different.
What's your current understanding of the situation and does it differ in ways from mine that you'd like to highlight?
1. https://www.fool.com/earnings/call-transcripts/2020/02/13/re... 2. https://www.elsevier.com/about/elsevier-and-open-access 3. https://www.elsevier.com/connect/learn-more-about-elseviers-...