Ward's only purpose was to injure Gronkowski on that hit. He went low on purpose to take out his knees. There is no excuse for his actions regardless of whether the NFL calls it legal or whining by defensive players about high hits. He intentionally went at his knees. Zero class on his part.
As for the PI, the refs ignored other PI's earlier in the game that were blatant by Cleveland. On the final play, the DB grabbed the left arm of the receiver as he was trying to extend for the catch preventing him from reaching out for the catch (arm bar). Obvious PI.
Spoken from your fan side, not from your thinking side. They lucked out and won again (and a squeaker against Cleveland - really?), but with or without Gronk, I don't much like their chances in the playoffs.