What kind of plus would Anaheim expect for the deal like this:
Detroit
Tatar
+
Anaheim
Theodore
cap dump (Ducks would need cap to resign Tatar in the offseason, he is RFA)
Zero interest in moving non exempt assets like Theodore for players requiring protection. We have one protection slot open for a forward, and need to move a top 4 Dman to protect the other three. If we make a move like the one you've proposed, we've handcuffed ourselves to accepting exempt assets only when we go to trade a Dman.
Not only that, but we're counting on one of our big three D prospects to take on the open top 4 slot next year and have yet to determine who it's going to be. I really don't think we should look at trading any of them until we decide who's best to keep. The situation still needs to play itself out, much like our goaltending over the past couple years.
So in summary, Ducks fans almost universally agree that Vatanen is the guy who should be dealt before expansion, and none of the three top D prospects should be moved until one of them grabs the open top 4 position. That's just the reality of our situation, if Vatanen doesn't work you will likely have to look elsewhere.