Great question! You're right on both. Old line is more likely to maintain the form and curvature of the spool it was stored on, meaning it will be more likely to twist and tangle after loading it on the reel spool. Your other point is even more important. Line loses its strength over time, and ten years is very old in fishing line years. Storage conditions can help maintain the life of fishing line, especially if kept out of direct sunlight and temperature extremes, but even in the best conditions, I'd say 10 years is too old. Old fishing line will break much faster than a new line, and losing fish on broken lines is terribly frustrating.