Robben was injury prone and surplus to requirements. No one thought he would blossom at Bayern. Alexis was good, but not outstanding for Barca. They got Neymar and sold Alexis for nice money. Fabregas the same, never arrived at Barca the way people thought or he wished. Thiago forced his way out, same for Kroos. Both last year of a contract and wanted out. Kroos wanted more money and Thiago wanted to play. Pique wanted back and wanted to play, had Rio and Vidic in front of him. Sir Alex and the club were very nice to him and let him go for cheap money, as our club did at that time with a lot of players.
Varane is injury prone and not better than Pepe and Ramos. Real wont sell him for cheap if they decide to sell him. He has a contract til 2020. All the players you listed had different situations than Varane now. He is their 3rd best CB and Zidane likes him. I dont see it.