It's not that bad a move. Minkah was a #11 pick, and most critical of all the Steelers know they're not getting a bust. They have plenty of film of Minkah performing well against NFL caliber competition, rather than the relative crapshoot that is college football.
Second, it's a financial bargain. Because of the way rookie contracts are structured, I believe I read that over 65% of the total contract has been paid. So he's a real bargain on top of it, and you have 4 years (including the option) of team control.
I think for the Steelers, it's a good deal. For example, even if it's the #5 pick they traded away, the received the #11 pick Minkah and the other advantages of a known quantity and cheap control for 4 years.