On the other hand, there is little that necessitates this sort of thing to be part of the OS instead of just being separate piece of software like VueScan
This isn't the MS-DOS/PC-DOS era where it was necessary for software to have full and direct access to communicate directly with the (ISA 16-bit bus attached, whatever) hardware and peripherals on a desktop workstation, because that can crash the whole multitasking OS.
In the case of the scanner in the original post here which appears to be from the era of MacOS 10.4, 10.5 and 10.6 , the operating system clearly doesn't let the scanning software talk DIRECTLY to it, there's an abtraction layer of some sort for the SCSI or (Firewire 400?) bus.
