Bryce Harper projects to be a 5 WAR player. Donaldson+McCann+Muk project to be 6 WAR players total.
So that's the simple part.
Then add the risk associated with a long-term commitment.
Plus the fact Bryce is going to make an AAV north of the 31M you are assuming.
Plus a good chance we get a draft pick from losing Donaldson.
And your question answers itself.