McKinney is a jack of no trades. He does nothing well. Can't hit for average, power is not great, can't field, can't run, etc. His minor league numbers when his BABIP wasn't .350 were pretty average to below average across the board. Three teams weren't going to give up on him before his 24th birthday if he was any good. Known s*** taking up a 40 man roster spot versus potential s*** that is years away but might have a 5% chance of being good is an easy decision for a team that was on the verge of scorching earth. McKinney does nothing for this team. If Luciano was in the minors and was putting up decent numbers in A/A+, he'd be a trade asset. Maybe not a huge one, but better than McKinney.
If anyone thought McKinney was actually a 2 WAR caliber player then sure, that would be fair. That was a stretch though. I think Teoscar has a better chance of being that type of player than McKinney does, and Teoscar's just as much a long shot.
I'm not saying Happ for a low-A ball lotto ticket would have been a good trade, I'm saying it would have been better than getting what we got. Like I said you could sell me on Drury being a buy low candidate (even though I didn't see the point of acquiring him if we were rebuilding), but at least attach him with even a 45 FV prospect or someone with some upside. Putting McKinney in that category even at the time of the trade would have been a stretch.