This stage of development is: based on an OS that died quietly over a decade ago. Not exactly "new operating system", isn't it?

What a tremendously poor argument.

NeXTSTEP never caught on and was a commercial failure. macOS is based on it.

Netscape Communicator was open-sourced after it fell to Microsoft and later became Firefox.

Not everything that dies deserved to. Successors are not their predecessors. BeOS provided a good base for developers to work towards, so the project wouldn't get bogged down in trying to be "something new".