Well, to be fair to Turner, Turner and Happ are not interchangeable. Turner's really here to be a 1b platoon (primarily against LHP) and some deep 3b coverage. Happ neither needs a platoon hitter nor is 1b/3b option. Turner's primary role doesn't really change sans-Happ.
I agree that Turner's probably trending in a direction where the Cubs will have to make a decision on him at some juncture. He's still in a sample size where if you want to believe it's a combination of slump/sporadic playing time you can believe it as he's still sitting at 70 PA's over 1.5 months. I know the eye test looks like something else, but people were flipping out over 92 PA's of bad Swanson (30 wRC+), it's not like 70 is some impossible tailspin.
As of now, the Cubs still have a vested interest in figuring out what they have in Turner and hoping it's a slump versus being toasted. Especially because I'm not sure there's going to be an easy trade target for a bit. Until then, it's either Turner or like a rookie such as Jonathon Long.