Toni Kross is simply one of the games beautiful mysterious just like Andrea pirlo of Italy who was totally a genuine just like Toni Kross. I have really enjoyed his presence in the game and would have loved to see him continue but just as you said, this particular feature of players coming and going is one that makes the game so unique and for Madrid all what they can do is simply to adjust and look for someone to fill up that space.
Kross will retire happily in football, his memories in the history of football will be forever remembered both in the club side and the German team, he is just like other legends like Pepe, Ronaldo, Maradona, Pirlo, and other respective players that performed greatly for their teams and country.
He won Champions League in his last season, and he may win another trophy based on their performance in the Euro this year.
After Real Madrid manager Carlo Ancelotti in his recent interview said that he has told Toni Kroos to give him a call immediately he decides to return back to the club, many people were of the opinion that Toni Kroos might likely continue playing professional football after the UEFA Euros Championship which he has already announced will be his last competition as a professional footballer but for me, I was very sure that there's no turning for Toni Kroos as far as professional football is concerned. The Germany international have achieved everything that's expected of a professional footballer and have engraved his name in the history book of he beautiful game for a player that's won the FIFA World Cup and the UEFA Champions League title in six different occasions, I don't think there's any other football achievement that make Toni Kroos continue playing after the UEFA Euros Championship.
Because of Toni Kroos records and how he entertained football as a sport to the global audience, he'll always be remembered for his excellent style of football and the football world will surely miss him when he leaves the stage.