Actually, you can use virtually any version of the Mac OS, as well as various other flavors of Unix written for the Motorola or PowerPC chip, as the case may be. I confess I'm not sure what, other than the Mac OS, is available for the G3/4/5.
The hardware sales support the OS development effort.
Fine, but I'm not speaking about motivation; I'm speaking about the ability to do it. If it is a motivation thing, what's holding the companies back, since such a system would have the ability to immediately compete with Windows?
When I said the hardware sales support the OS development effort, I meant financially. OS development is not cheap; serious cash flow is needed to furnish the ability to do the work. There are a *lot* of highly motivated people working in Linux now, but most of their companies seem to be rather sparsely financed.