Meaning more miners will quit as the price goes down. In the end, a year from now only miners with zero electricity cost m ight be left.
Price could go down to perhaps $1-$2 in a year from now. Only when production of coins start to dry out, and a few years after will we know the final price.
You are ignoring operation scale; obviously people who are making loads more money mining are mining in larger volumes where it would impossible to steal electricity without anyone noticing. Someone with a quarter terahash/s operation will easily outcompete these free miners because their revenue is so much higher and they will have greater capital to invest in future mining operations, especially as difficulty increases, and will receive better rates because they are using hardware that utilizes less power (ASIC mining) or better deals on power because they are consuming it in bulk. Free miners with small operational expenses will be more likely to bail as time goes on, as the market will swamped will large-scale miners operating on 5-10% margins from ASIC/hardware specific mining. If the free miner buys a video card, mines all year with it and only loses money in the end because of the devaluation of the video card, there will be little to no incentive to mine even with no electricity cost.
In short if an ASIC miner can mine at 1/10th the power consumption of GPU mining, I would estimate the collapse of small-scale GPU mining (free or not) in a couple months time. This will only happen after extended stability in the market and is probably not an immediate concern, but sometime next year folks who are small time mining should probably be concerned.
That is, the GPU mining market will eventually be outcompeted by people who have the revenue to invest to do it more cheaply.