There r actually a lot of young and very highly rated CB's in Germany.
1st category r IMO players like Ginter (19, Freiburg), Rüdiger (20, Stuttgart) and Tah (17, Hamburg), but there r some more who r already starters and have shown promising performances like Knoche (21, Wolfsburg), Süle (18, Hoffenheim) and Brooks (20, Berlin)
Then there r some talents being squad members, but not being good enough yet like Akpoguma (18, Hoffenheim), Ayhan (19, Schalke), Sarr and Günter (18, 19 both Dortmund) or Wein (19, Bayern). Kempf belongs into that category.
He isnt a starter and has been given just a few chances, where he didnt convince at all and has shown not much to say that he is ready and stable enough. It's very unlikely, that he would be an instant help for a club of the stature of Juve and 5m would be definitely too much IMO.