I have been hearing that Bitcoin will never drop to zero ,because if that happens people might buy entire bitcoins and bitcoin will finally come to an end and that would the greatest regret ever in the history of crypto currency,but what happens if it drops to zero?will someone be able to mine entire 21 billion ?
I've seen altcoins dropping to 0. Those usually got abandoned by devs and delisted from the exchanges before getting to 0.
Do you foresee this for Bitcoin? I don't. Bitcoin still has strong development team and more exchanges get born every month and all of them start with listing Bitcoin.
Now about your confusion regarding mining. Mining is not related to value or price. Let me explain. If infinite cheap energy source is found, the price of coal will drop significantly. Maybe close to 0. Will this make coal mining easier? Will one miner be able to get it all? No. For Bitcoin it's somewhat similar (even if only one miner will remain to mine Bitcoin, he will still need many years to come to mine all the Bitcoin because the software balances it and the new blocks will come at approx the same distance in time). So, mining is not related to value or price.