Dortmund lost players to Real and Bayern though, historically great clubs. I guess Chelsea are a big club too though with the money, so many CL semis, etc.
Atletico will want to sell if the price is right given their debts. Simeone sounded confident earlier in the season of being able to replace Costa. If they're able to keep the rest of their squad I think they'll still be in decent shape, but if Chelsea grab Courtois, or somebody comes after one of their defenders, or Mario Suarez or Koke, then things could unravel a bit Dortmund-style.