I didn't foresee any relegation struggling team win the premier league the following year until Leicester pulled it off. Until these things happen, no one ever sees them coming.By experience, I am referring to them being slightly raw in the defensive sense. Little things come over time. I don't see any top team having two 21 year olds as their main centre back pairing.
That said, seeing as Hummels and Subotic were both 22 turning 23 when they stormed to the Bundesliga title in 2011-12, it doesn't strike me as such an out of this world feet. But regardless, we have Komps and Otamendi, so I don't reckon this will be an issue.