Were you there at the time? Because I was a big computer nerd at the time, huffing all the OS/OS fumes I could get my grubby little hands on. Windows 3 had already won the game -- and that was when non-computer-nerds were asking their computer-nerd friends for advice and getting PCs hand-built by the same. When win95 came out, the non-computer-nerds forgot that the command line existed. When win98 came out, even computer-nerds were losing interest in the command line. Win2k was (imho) the best windows operating system ever released. It was extremely stable and usable, supported everything but apple software and a few bits and bobs that nobody but us nerds cared about, and it took serious effort to buy a computer that didn't have it installed by default.

So a year after win2k is released, your selling points are "basically usable" ( vs "highly compatible"), "free/[nerd-shibboleth]" (vs "hidden in the cost of a computer"), and "easy to install" (vs "already installed"). I think it's hyperbole to suggest that BeOS being open source would have dramatically changed the course of computer history. If anything, I think it's worth considering what would have happened to the already-paltry Linux Desktop experience if BeOS absorbed developer attention.

As someone that has the CD-ROM they shipped on magazines as advertising, it was mostly usable, for single users.

And after they lost to NeXT, regarding being acquired, not much else happened in regards to OS development.