There is no minimum amount of hashes that one needs to produce to contribute to Bitcoin mining. It's totally parallelizable.
Nobody is going to run a machine for a few cents or dollars, or run at a lose of electricity cost. Even if they run a small unit just to sort the network, their 10gh is nothing compared to THs and PHs large miners will deploy so end of the day, your small miner becomes smaller and smaller share of the network.
I'm doing it right now, because I need heat in my basement anyway, and electric service is 7.5 cents per KWH where I live. That's about the same as the cost of propane, which is my other choice. So why not, since I have the rig?
Can't compare us to the rest of the population, who will never run a miner.
We don't need everyone to run miners.
Secondly you running that 1 rig doesnt matter. As the network grows your share of the network decreases for people to keep it decentralized they need to keep adding hash as the network grows. The big guys will keep getting bigger while the small will get smaller.
So? Do you think that wasn't foreseen?
EDIT: When we say that Bitcoin is decentralized, we mean that the network has no 'center'. There is no node with more authority than others. Massive mining centers could be shut down by fedco, but so what?