He kind of is the guy. His offensive production right now, on the downside of his career, equals the person he replaced at 3B at his peak (OPS+). His defense is unquestionably better over there, too, even if he is not perfect.
Injuries suck, but his games played is not far behind the league qualifiers.
What's wrong with 25 home runs? (I think your estimate is high, mind you, but I would take 25 home runs from almost anybody).
I would not have signed Donaldson, and I admit I'm still very questioning of his future. But hey, he's doing his job. The Twins goal with signing Donaldson was probably two-fold: minimize the offensive drop-off at the position from Sano (check), and improve the defense (check).