One definite workaround is to write new software and release it under a free software license such as GPL. From what I understand, it is even perfectly legal to reverse engineer file formats which allows, for example, Open Office to legally read and write real Microsoft Office documents.

Strictly for personal use, you can likely get away with using actual unlicensed software on ancient computers. For distribution or commercial/professional use - it is perfectly reasonable to make the investment in producing your own version of the software. It may even be possible to start with modern code that's freely available and port it backwards to Mac OS 9, 8, 7, 6, etc.