Seems odd to me... the Dacron in sailcloth, especially if kept out of UV exposure, lasts far longer than that. I've had sails
that were over 20 years old that were still serviceable. They had spent most of that time in sailbags, but had been exposed to salt water
and sunshine at times.
Perhaps the Dacron used in fishing
lines is different, but that seems unlikely. I would expect t hat the basic fibres that both line and cloth yarns are spun from would be the same. Post spinning they might experience different treatment... I dunno.
I'd be pretty upset if a sail fell apart after five years!