*A company like Yamaha or Native Instruments creates a computer designed solely for music...
Yamaha did that years ago.
True, but the CX5 was designed to compete with things like a Commodore-64. In a doomsday scenario, there wouldn't be
"other" computers as we know them...if Yamaha wanted to sell Cubase, they'd say screw it, let's just make a computer. That's what pretty much what all companies are doing for instruments, with embedded systems.
Also remember that back in the days of CX5, it was far more difficult to assemble your own computer. Today, making a PC isn't much more difficult than it was to assemble a model airplane.:) Yamaha doing a computer would be similar to Sweetwater doing Creation Stations.