Ancelotti is a living legend and for him to have accepted the offer as the new coach of the Brazilian team means that he's going to do his magic to bring back some of the great players which are not part of the team yest to ensure he builds the team with them. Ancelotti is a great coach he will definitely use his experience to achieve a good result to the Brazilian national team.
He doesn't have to accept the offer, at the best I can think of Real Madrid had planned to change him from the beginning, so it's not a new things to see him been change now, who will leave a big paying club to go and settle for a national team, no way Brazil is going to give him a full contract of huge payment unless he deliver something first before they give him that bread. It's obvious Real Madrid had him for change before he switched quickly to Brazil.
He has done his best I must say, I'm not sure if there is any coach in that coach Real Madrid in 2000s that has long time like he did in Real Madrid and when he eventually did, the timing was worth it, he had an incredible trophies for the club and has made name for himself with personal award. If he decide no to work again, he be fine as I even learn that he will be receiving some pay after leaving Real Madrid for good which is good if that's true.