"Intellectual Property Maximalism rollback: copyright terms have not and probably will not be indefinitely extended again to eternity to protect Mickey Mouse, and in 2019, for the first time since 1998, works entered the public domain"
I think the easy indicator may be the wrong one here. Defined more broadly, the public domain is not being enriched. For example, the web was a lot smaller in 1999, but it was a much more public domain. Today's web and post web internet is more centralised, controlled and therefore private property. Google could crawl pages, links, forums, because they were public, and use that access to create a search engine. Content, connections and signal are, today, proprietary. You can't order the world's information if that information is facebook's, only facebook can.
Or patents, more stuff of the last generation is patented than the previous'. Does that mean we invented more or we patented more? What happens to stuff that doesn't get patented? It's public.
Old copyright expiry deadlines might be a symbolic lead indicator, but they're determining the location of a fence post in county scale land dispute. A tiny, legible, part of the whole. In real terms, Disney's copyright portfolio is worth more, not less.