Satoshi thought and planned for it.
If the bitcoin price falls below the cost of mining, some miners will exit. It will temporarily cause block discovery to lengthen as the existing miners striuggle - but after 2400 blocks, the difficulty will adjust down. That means it's easier to mine. (The difficulty is directly linked to the cost of mining as you need many more miners to win a block when the difficulty is high).
Basically you'll get iterations of this till the cost of mining falls below the bitcoin price.
If the bitcoin price fell to $1 I expect you'll be able to mine them with your laptop as the professional mining farms will have long given up. But when would we see 1$ price again, If we really did see that price again I would definitely start mining bitcoins and believe that it would generate me a lot of profit in the future. Nevertheless, It is nice to see that satoshi have thought of a solution before vanishing out of nowhere. But what will happen if all bitcoins have been mined? Wouldn't that be very unprofitable for most of the miners out there?