Why won't they just open source the reader instead of abandoning it? I would love to have at least one 100% working epub reader on Windows...

SumatraPDF is fine to read EPUBs on Windows (also supports PDF, MOBI, FB2, CHM, XPS, DjVu) - it's free, open-source software [0][1].

[0]: https://github.com/sumatrapdfreader/sumatrapdf

[1]: https://www.sumatrapdfreader.org/free-pdf-reader.html