Not many cap dumps help a rebuild. It's either an overpaid guy with term, or a high AAV guy with less term that a team just needs to unload for immediate flexibility.
The former probably nets you something of significance, because few teams want to take on a player that could become an anchor down the road. The latter won't hurt much but also isn't likely to get you a sweetener if the guy can play at all, since many other rebuilding teams could easily take that on too with little issue.
Plus this frees the Ducks to deal both Dumoulin and Fowler without leaving only one veteran D on the team for this year and next. Both are likely to be desirable TDL targets.