Everything gets worn over time and that includes every type of fishing line. Despite being very strong, a braided line is not going to last forever and will need to be changed over time. Monofilament lines deteriorate but only very slowly and can still last you for a very long time.
With a fishing line there are many things that will wear it down over time. One of them is the environment that they will be in, especially if they are in saltwater. Another is just the general stretching and usage that it will get as it moves around the reel, through your rod and against anything that it might touch. Another thing that can deteriorate a line is the natural abrasions that can happen underwater from numerous sources.
No-one is impervious from the wearing down over time but braided lines do have an impressive amount of resistance to everything that can be thrown at it. The reality is that if you choose a line from a reputable brand that is well-made then it will last you for a very long time, regardless of whether it is braided or a monofilament line.
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