It's true, there are no eternal players in football, what happens is that new players will be born and old players will retire, and so on. However, what makes the difference is whether the player will retire when their abilities are still very good and are still very much needed, or they will retire when they no longer have the ability to play. And Toni Kroos has decided that he will retire while his abilities are still very good and are really needed by his club and country.
It is quite difficult to find a player like Toni Kroos, but it is not impossible that there will be a new player born who will replace Toni Kroos. This is a very normal thing in football, because we cannot expect players to be able to play forever. Many star players have to retire and we have seen this from generation to generation. There is always renewal, therefore the regeneration process will always exist.
If he felt that it becomes more difficult for him to maintain such a high level of the game as before, then it is better to leave when everyone will remember you as best of the best. Moreover, he did not completely leave football, he will continue to work in Real, and will train young players, and perhaps once decides to try to continue his career in the role of a coach. He has every opportunity to leave football in his life as long as he wishes.