Mhhhh, have you actually played the junk turret ? Because I have a 65 hour pure intellect playthrough going right now (well, not pure anymore, but I've been for a long time), and my reality couldn't be further away from what your post tries to lay out. Yes, turrets do work, especially once you have 2 of them with 5 points into their perk. Do they need tweaking ? Sure ! Are they broken or really that superior like you pretend them to be ? Hell no, it's quite the opposite unless you're a totally beginner in first person shooters.
First of all they are situational compared to the other weapons. You need an area that has some space to it, and very often with the way sleepers work (waking up when you walk into the room), that means backpedaling a bit, planting your turrets, and waiting for the zombies to come towards you. Not only is it cumbersome and does it take thim, but this is all good unless you get to a high enough gamestage where a majority of the zombies are runners. There's obviously always a "way" to make them work in any POI, but it's very often way more annoying compared to just clicking heads with any other gun.
Also, the benefits skill points regarding the turrets are off. From rank 4 to rank 5 you magically more than double the effectiveness your loadout. That alone feels very weird, as in "it's either OP rank 5 or it was trash before". Then, the shooting vs placing dilemma : the dismemberment chance from your Intellect points does NOT proc when your turrets are placed down, but shooting the turret yourself is insanely slow compared to when they are placed on the floor (and very imprecise).
You mention the fact they should only be active when you're 1-2 blocks away. How does that solve the "closed door problem" you're talking about ? Can you imagine how annoying and static the gameplay would feel with such restrictions, especially when using 2 turrets ?
Lastly, what you call an "exploit" is really situational. Dropping down turrets 2 floors beneath to clear a whole floor with zero risk ? How does that actually happen ? Your turret needs line of sight to shoot zombies, so unless we're talking very specific conditions where there is one single room and no obstacles that prevent line of sight, I don't see that happening nearly often enough so that it is a problem. Not even mentionning you then need to go down and pick em up... And "planting your turret and closing a door" is really no different than whacking out a single block and shooting them in the head while they try to bash their way in...
Don't get me wrong, junk turrets can clear out zombies very fast if the conditions are right. Sadly, unless we're talking 7th day horde, those conditions are filled way less often than the ones needed to wipe a room clean with any other full perked gun. And as a matter of fact, clearing a POI with my 0 perk level 4 AK47 was way more of a breeze than using 2 level 5 fully perked junk turrets...
