That's not what happened. They kept him around after signing Tavares, they thought they would have this amazing luxury of having three top centers down the middle (Tavares, Matthews, Kadri). They traded Kadri because he was their best asset to land a defenceman at that point, and because Kadri then got himself into trouble once again in the playoffs. Kinda like Keefe being fired today, it's not about not liking the player. It's just that the situation became untenable.
What ruined them was going after the big name UFA center and handing him top dollar without realizing that they have two rookies on the roster that will become better players than said UFA player before their ELCs are up. And these rookies and their agents are going to take that into the negotiations instead of being pushed around like rookies 10 years ago.