Positron was a completely logical step in the evolution of idexed shifting: it put the detents for the different grears on the derailleur itself. Unfortunately, this requires that the cable push as well as pull, and this doesn't work well with fine-gauge wire. Pull-only systems allowed the use of lighter and more flexible (and non-proprietary) standard shift wire and housing with the detents moved to the shifters. Front Freewheel is a fine system that was handicapped by being offered on low-end bikes built to a price point. But it moves the bearings a little further outboard, and had the cranksets themselves been able to take replaceable rings, it very well might have caught on.