It would have been a disaster, BeOS was significantly different from a NeXT/BSD-like unix type base that made it relatively easy for software to be developed on MacOS in the 10.0 to 10.4 era. Early versions of MacOS X like 10.0 were clearly just an evolved NeXT with a cute MacOS finder like skin on top.
Well, not only former Be but former Finder developer here. The OSX Finder was certainly not a cute Finder-like skin, it was in fact huge amounts of the OS9 Finder source code including a complete copy of a modified Metrowerks PowerPlant C++ framework underneath it all. There was even elements of the old MacApp framework there. It was not a skin; it was a source code behemoth with massive amounts of legacy code.
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