The D6 still makes a living. The 7 is in retirement but still gets up to work once a year to stay limber.
The shop I work in was a old crawler shop, RR rails built into the concreate floor. At one time there were more D4, 5 and especially D6's in this area than anywhere. The hilly ground around here every farmer had at least two, three or four crawlers. Getting pretty thin these days with rubber band machines every where.