There will never be a situation where no miners mine. First of all, bitcoin would collapse in security, and second of all, I'd just sit at my job and CPU mine and make 300BTC/hr. That's why there is no hard limits - free riders on one side, barrier to entry (expertise, OpenCL-ready graphics card availability) on the other.
I think you still missed something I am trying to get across.
The hard limit I stated is for the current difficulty. Now in reality, if that limit is reached (BTC value is below electricity cost to mine), when miners start to mass drop-out, difficulty will go down in the next adjustment and everything will be fine. Also, many miners will drop out early, before the hard limit is reached, because individual energy costs vary and willingness to mine for little profit isn't universal.
The hard limit simply means everyone would drop out if reached this point. Reality is that most miners would drop out before it is reached, and furthermore reality is that as these miners dropped out difficulty would go down before the hard limit was hit and adjust the limit.
As far as the "other side", the barrier to entry you refer to doesn't exist because it doesn't require new miners to start mining- existing miners can simply add additional mining hardware, if profit is high enough. It doesn't matter if x% of bitcoin users will never have the capability to buy even a cheap mining rig. If it becomes profitable enough, mining difficulty WILL be pushed up because of existing miners who add additional capacity.
Although, just like the lower hard limit, the upper hard limit will never really be reached, because some miners are willing to add more capacity at lower levels of profit, which in turn will push up difficulty and keep the hard limit from ever being reached under normal circumstances.
If BTC drops below $2/BTC I will still mine because it doesn't cost me anything to sit at my work computer and run a mining program.
I covered this too. A few irrational people would mine even knowing it'll be at a net loss. While it might be a gain for you, your employer is losing electricity, and overall it's a net loss.. You might as well steal money from a bank and use it to pay for your electricity. But again, this wouldn't occur because the market would adjust before hitting this point.