Along with the aging players and loss of mentality from before, losing players like Arda and Costa hurt them a lot as well. Despite his form here Arda, was a key player for his side and fit the role as a Wide Midfielder/ Winger very well. Costa was a machine for Simeone and fit his system so well.
they spent so much money on trying to replace Costa with Gameiro,Jackson Martinez,Vietto,Raúl Jiménez,Mandzukic and going by the Lacazette, rumors Gameiro didn't satisfy Simeone enough . Torres was free obviously and no one should expect much out of him. They got the Griezmann part right but no one really to fill the Costa size hole that was left.