There's not much hope for a commercial software product if the makers have given up on it, unless they decide to open-source it, which has happened on rare occasions.

The story illustrates one of the advantages of free software, and especially of open standard for file formats - if the file formats for Sibelius were documented and available, someone else could write software to use them, and "decades of work invested in it" wouldn't be a problem.

Can Sibelius export to a different format (ABC, MusicXML, Lilypond) which can be used by other software? If so, that might be an escape route.