If all/lots of the miners pulled out, then wouldn't the work factor decrease so that regular people could run it on their desktop machines? That is what happened at the start of bitcoin, right? Are miners that required?
Because that difficulty factor changes over many mined blocks, if the ASICs all suddenly stopped mining that chain, there might not be enough hash power to ever find another valid block.
And until so many blocks are found, the difficulty won't change, so the chain will be fully and completely dead (at least on the scale of months)
For a rough idea of the scales, just look at the current reward for finding a block (12 btc) and it's current value (about $31k USD). Miners are just barely breaking even on ASICs, so a $31k miner should be able to pay for itself over it's lifetime.
So unless you have hundreds of thousands of dollars of miners, you aren't going to realistically find a block, and unless you have millions of dollars of ASICs you aren't going to be able to mine enough to keep a chain from dying on your own.
Yes, we need miners. But yes, you're right, the difficulty is adjustable, so ultimately either we need miners[0] or we need miners[i].
If there's any value to be had for a coin and the PoW matches existing hardware, it's likely that miners will show up.
other people would start mining-- botnets, gpu gaming enthusiasts, people with excess FPGA capacity, etc. and then these people would become miners.
Some alternative crypto currencies use algorithms that supposed to be really hard to do in ASCIC, I suppose those would be more feasible for rel people to use them, but as I understand we are stuck this way with bitcoin.