There's currently a reddit beta app I'm testing and the dev has claimed to have found a way to implement what he calls "2.5D Touch". This is actually brilliant in its own way but also a bit funny because it's kind of like a knock-off that works wonderfully. Basically, to give users without a 6S a 3D Touch experience the dev has made it so when you press down on the screen the app detects your initial contact and later the spread of your finger on the screen to know whether you want to tap or peek. So, when you press harder, your finger spreads through more of the screen, therefor letting the app know you want to peek. I'm sure that Apple has the willpower to easily implement this system-wide, however, it would not be in their best interest as 3D touch is currently the only real feature the 6S has going for it imo.