Context for non-dweebs: Unlike Tolkien's other well known works, The Silmarillion was published posthumously; Tolkien died in 1973, and The Silmarillion first saw print in 1977.
Though Tolkien had shown drafts of The Silmarillion to publishers during his lifetime, there are substantial differences between those drafts and the book that was actually published. It's been a matter of great interest – read: nerd drama – in the Tolkien fandom exactly how much of the published Silmarillion is really the work of J R R Tolkien, and how much of it is original authorship by his son Christopher.
The Tolkien estate has historically maintained that The Silmarllion is all J R R Tolkien, and that Christopher merely acted as an editor, because "by J R R Tolkien (edited by Christopher Tolkien)" is going to sell better than "by Christopher Tolkien (based on the work of J R R Tolkien)".
If The Silmarillion really is 100% J R R Tolkien's work, and Christopher Tolkien was merely an editor, then – since J R R Tolkien died in 1973 – the whole thing will enter the public domain on January 1st, 2044 in all life-plus-70 jurisdictions (i.e., most of the big ones, including the US).
If, however, any major part of the published Silmarillion constitutes original authorship by Christoper Tolkien, then the term of copyright would instead be calculated based on his date of death in 2020, pushing its earliest possible entry into the public domain in life-plus-70 jurisdictions back to January 1st, 2091.
Thus, there exists the possibility that the Tolkien estate might be able to preserve their ownership of The Silmarillion by arguing that they've been lying the whole time about Christopher Tolkien not contributing any original authorship to the published work.
Would it work? Probably not – but it'd be fun to see them try!