It has been quite a while since I've heard the CD12, but, it at least was a terrific sounding CD player at that time. The fact that it is old does not necessarily mean its sound is out of date. A lot of current players purposefully use older chipsets in their DACs because, in their analysis, these older chips do a better job than the latest chips. These older chips have actually become quite valuable for this reason (some examples are Zanden and Naim).
Improved audio quality is NOT the driver when it comes to priorities in "improving" newer chips. The main concerns are adding more functions (not just digital to analogue conversion), smaller size, lower power consumption and lower price.
However, the other concern is availability of replacement parts. I hope Linn kept enough spares to support the CD 12 for a long time. I would expect Linn to have done this because they appear to be a company that thinks about the long run and about keeping the same customers for a long time.