The way this piece of history fit into Commodore managerial logic is described pretty well in Bagnall's book. I don't have it in front of me, but politics and licensing drove a lot of basic architectural choices in the 65xx machines.
It's got Herd in there describing the rough moment when someone realized 'Doh! Compatibility would make sense!' (Somewhere around the Plus 4, and lo, the next major release was the 128.)
