Some of even the biggest stars ever demolish their legacy by playing way after their peak (hello Cristiano, hello Lionel) but he is going out at the very top and you have to respect that.
Bad snobby take I've heard from Kroos and a few other of his fans about "looking so good by retiring at the top". Good on him but nothing wrong with playing until your late 30s even if you're past your peak.
Neither Ronaldo or Messi have demolished their legacy (even with Ronaldo's United antics). Good for them for playing in lower standard leagues outside of Europe where they can still show off their prowess , and cash in on their brand and give fans outside of Europe a chance to see them. They are entertaining generational players so the longer we get to see them, the better. It's not like there is much for them to offer once they retire.
Both played at Ballon D'or winning level at Kroo's age, even Benzema and Lewandowski did at the same age so it would be a waste for them to retire at 34. This doesn't apply to Kroos as much because no one is going to be too bothered if the master sideways passer retires early.
Modric has a higher chance of demolishing of his legacy. There were reports the old man wasn't happy to be second fiddle to the youngsters during the season. It won't matter now because Real Madrid have been successful but Modric wasn't key for them this season.