Nobody is going to pay for mining rigs that are idle 99% of the time.
If it happened enough you'd have people set up cheap resistor banks to eat all the excess, so that provides a natural cap on how much negative pricing there can be.
While subsidies are one cause, another cause is demand response problems:
Imagine you have a greenhouse factory producing strawberries. The strawberries come out of the factory on a conveyor belt. Now, you can't just turn off this factory easily since it might take a week to get it up running again and you don't have any storage, so what happens when nobody wants your strawberries for a day?
You pay someone to take them.
And yes, it happens to physical products too, e.g. household electronics - you need to pay someone to scrap them.
A better example would be plastic production. In an emergency stop, factories need to change several pipes because the plastic solidified inside. Stopping production properly takes days, and even then, resuming it also takes days.
This happens when the electricity supply is higher than the demand. It only affects the wholesale rate, not consumer rates (at least in Uk and US)
Normally supply and demand need to be matched to obtain a balance. If the supply is not enough for demand then the frequency drops below 50Hz, if it drops too low bad things happen.
The renewable generators get a subsidy for each kWh that they pump on to the grid. The logical thing to do if electricity were oversupplied, i.e. you have to pay people to take it, would be to temporarily shut down some renewable generation, but then the renewable providers would lose out on the subsidy for those kWh.
But Tesla is reinvesting a lot. And it's growing the market for electric cars, setting up infrastructure, etc. He might be selling at a loss initially in order to create the volume that will eventually drive down the cost so he can generate a profit. From what I understand, the big reason he keeps losing money on selling cars is that by the time the previous car starts making a profit, he's already already started selling the next one at greater volume, creating new loss that drowns the fresh profit from that previous car.
Is it possible that Tesla is driving down the cost curve faster than any other electric vehicle manufacturer, and once they hit a tipping point, they will disrupt the entire automotive industry and be worth 10x their current price? I may not believe Musk’s self driving car vision is attainable in the gritty real world in under 10 years, so I’ll discount that... but driving down battery cost, while making a better electric vehicle than Audi/Mercedes/Porsche.. that’s exciting.
Edit: also why don’t they just mine some coins?