Whenever he had to replace a player of the old-team (Filipe Luis, Juan Fran, Gabi, Arda, Costa, Tiago, Miranda-Godin), the new player ended up performing worse. he only replacements, that improved the team were Oblak, Griezmann, (David Villa for a short period of time) and players who got their breakthrough at the club. He had players like Mario Suarez, Raul Garcia, F.Torres or Adrian Lopez, who were hardly world beater, but still managed to contribute. Nowadays the bench is just continuing to disappoint.
They bought many interesting players and most of them under performed. That would speak for a new manager.
On the other side, Atletico don't have solid finances and missing CL could be a disaster. They can't afford to gamble and Simeone guarantees CL football. I don't think any other manager can guarantee this level of stability. Its hard to believe that Simeone can manage to lift them to the absolute top level again, but their management would be still crazy to sack him as long as he is happy to work for them. This might not be pretty and its certainly disappointing to see some of their wasted potential, but I don't think they can afford to take the gamble on a new manager. The downside risk seems way too big.