This is true, however the adjustment period it is still vulnerable to manipulation. If I spent 100 million USD on research and development of mining equipment, and I was in sole control and custody of the machine. I could easily disrupt the mining process by mining blocks selectively. Lets say I crash the party and start up my miner at the beginning of the block, I could easily find it within (for the sake of the argument) say 10 seconds. The difficulty would change tremendously in order to account for this increase of mining speed. Then lets say I turned off my miner, the other mining equipment could take a while before finding a block at the adjusted rate. Essentially nothing is finite in the blockchain system, no matter how much everyone thinks it is. You can argue the date on when the last bitcoin will be mined, but given that this is the first of its kind my guess is still; the date is unknown.