In a world where Pinamonti is worth 20 millions and this Scamacca dude even 45 million, paying 10-15 million for Thuram is a good deal. He is still Borussia's best player in terms of market value (28 million, highest point was even 40 million) and that with having the likes of Zakaria, Ginter and Neuhaus in the same team.
I agree, he is no worldbeater, but that's is shown in the rumoured fee. He is fast, isn't shy of dribbling past an opponent, good at header, can win penalties (not like Lautaro with his stupid dives). Imho he could benefit a lot from Inzaghi and his teammates.
There is lots of upsides and even the downsides aren't too high. If he fails, we would get at least our money back by selling him to teams like Genua