I have and still use Rio fly lines that I bought in the early 2000's as well as more than a few SciAngler Mastery lines that are close in age. No single lines gets fished 100 times a year but after 20 years the number of hours on certain lines is quite high.
A couple of things I'm adamant about which may explain the longevity I get:
I clean my lines OFTEN with nothing but plain Ivory Soap & water and treat them with the line dressing recommended by the manufacturer. FWIW - Because I use Rio, Sci Angler, Wulff & real silk lines it means I use FOUR different line dressings.
No matter how many "experts" make videos extolling the virtues, I never stretch my fly lines if they get twisted or coiled. If necessary, I usually let them hang below me in the current to relax.
I always take the reel(s) I used last OUT of its/their case(s) when I get home and allow them to dry out overnight at minimum. Often for days if I use other combos.
I'm very careful about NOT stepping on my fly lines, especially with studded wading shoes. However I have more than a few lines still in rotation with nicks or cuts from carelessness that I repaired with the tube of Loon UV Knot Sense that is always with me.
I NEVER replace a floating fly line until it starts to sink regularly no matter how old it is, how ugly it has become or how many "micro cracks" I can see in the coating. If it floats, I still use it which may explain why I have 20+ year old lines.
I make sure long term storage of lines is always in a cool place out of direct sunlight.
Good lines are worth the money to me because of the performance, float (if it is a floating line), suppleness and lifespan.
I also know the money I spend on other fishing consumables EACH OUTING like gas, cigars, whisky & craft beer, a bit of tippet material and a few flies is way more than the cost of a good fly line over however long they last for you.