Pajero timing chain

Timing chains do not need to be replaced regularly as timing belts do, so there is no requirement in the service schedule for such a replacement. But they do wear and can become slack over time, and when that happens they do need to be replaced. If you don't replace them in that situation you risk them breaking or jumping teeth on the drive gears and if either of those things happened you could do serious internal damage to the engine. I'd be inclined to follow your mechanic's advice.