Matter doesn’t stretch endlessly, though. Think about the individual mithril atoms, there’s the same number but they have more space to cover as time goes on. The wire would gain some tension as space tries to expand it, and relaxing that tension could be used for power generation.

Or it would break. We can fudge something unnaturally strong but at some point the speed of sound in the material starts brushing against lightspeed and it just has to give.