That's not what you said.
You said "It's a good position for the new manager to be in. He built a decent young side at Ajax before the predators swooped".
He inherited De Ligt, De Jong, Ziyech, Van de Beek and Kluivert (who were all sold to "predators".). Neres was sold to Shakhtar. Onana is still there.
The players he signed that are in the article you've used to back your argument were:
Antony - 18yo when he signed. This is the only player who meets the criteria of being signed by Ten Haag and being young. He's still there (young but no swooping)
Promes - 26yo when he signed (three years ago). Was sold to Spartak Moscow for half of what they paid for him a couple of years later in 2021 (not young and no swooping).
Haller - 26yo when he signed (last year). He's still there (not young and no swooping).
Tagliafico - 25yo when he signed (four years ago). He's still there (not young and no swooping).
Tadic - 29yo when he signed (four years ago). He's still there (not young and no swooping).
Blind - 28yo when he signed (four years ago). He's still there (not young and no swooping).
Basically, he signed one young player (Antony) and he's still at the club - so he didn't "build a decent young side at Ajax before the predators swooped".
Considering your OP was "who does the new Man Utd manager keep? I can't think of anybody" and then saying Ten Hag built a young team and "It's a good position for the new manager to be in" - I wonder if you can see the contradiction?
He inherited a team of promising youngsters and coached them into being great. He then signed some older players who did well too. If there's nobody at Man Utd who you think they should keep, it's hardly the scenario Ten Hag faced coming into the Ajax job in 2019, is it?
Enjoy the rest of your evening