That only works if you believe you CAN improve your squad in this window. If Wolfsburg don't believe they can, then it makes more sense to keep the money until they feel the right players are available. If they believe that (and they seemingly do), then it makes total sense to drag things out for a few more million. They've nothing to lose, as they've already resigned themselves to keeping the money for later. If the deal falls through, they keep their man, otherwise, they get the max price for him. Not every promoted team opts to spend a fortune because they've been promoted, and in the same way, not every team opts to spend a fortune just because they're in CL. There's nothing 'wrong' in accepting you're not going to win it anyway, so just enjoy being in it, and spend sensibly with any money they get.
If we sold Aguero, I'd not be happy with 5-6 players as a replacement (at 8m a go). I'd rather wait until we could get another world class forward, even if that mean waiting until next year.