There is no downgrade in the forward's work rate when you go from Costa to Mandzukic. If anything, it is an upgrade. Your reasons don't make much sense.
They are playing the same way as last year. Does losing the keeper and replacing Costa with Mandzukic, Griezmann and Cerci mean they go from top 2 to out of the top 5? I don't think so.
I think they are still better than PSG or Dortmund until proven otherwise.
Dortmund replaced Lewandowski with fucking Immobile and somehow now they are better than Atl. Madrid.
Mandzukic is quite a bit lesser compared to Costa, who had he impact that Cavani had on Napoli. Mandzukic is also a worker offcourse, but lacks the immediate danger that Costa brings.
You go very lightly over "losing the keeper". Courtois was probably even more important then Costa. He grabbed so man points for them.
Sorry, but an inferior striker, a reasonably good winger and Cerci are quite a step down from a beast like Costa and the best goalkeeper of 2013/2014 season.
But that isnt even the situation here.
Bayern got stronger, still significantly handicapped by guardiola tho
Madrid got stronger
Barcelona made good changes wich will lessen their drop in quality following Xavi's decay.
Chelsea got incredibly improved
PSG was better then athletico last year, and if Zlatan keeps that form and isnt injured when it matters, they are even more ahead this year
SO yeah, athletico isnt a top 5 team anymore. and they never were top two.