Or at least expand it onto the rest of its range? As far as I can work out, the only part that seems more expensive to implement in a DD mechanism is the coreless motor. Yet after the DD30 series and DD9, they stopped using it.

The advantages offered make it a great selling point. Very low wow/flutter, high reliability, no belts. The main disadvantages are: Difficult to implement to the "slim" Walkman designs, and expensive to implement for auto-reverse. Could this be the reason? Like today's iPod, it is harder to sell a product on technical specs alone, instead of what was "in" for Walkmans at the time: Slim, remote controlled, auto-reverse.

The only reasons besides those I can come up with is planned obsolescence, what with DD mechanisms being able to last for much longer if not for that center gear.

My guess would be the reason behind much of the 'decline & fall' of the boombox & walkman:
They found a way of producing the product *much* more cheaply, & never mind about the quality drop off.