General life expectancy of spooled line?

[RB 77]

Every baitcaster has braid backing. I retie my  mono/ flouro top-shots when they get too "short". Spinning reels have braid to the lip of the spools. Flouro leaders get replaced when they get too "short" as well. 

[Bankbeater]

With regular fishing the mono on my baitcasting and spinning reels need replacing about every 3 to 4 months. 

[king fisher]

I only replace braid when I get low on the spool, which is rare, 2 to 3 years.  Braid will last a long time unless fished around rip rap without a leader.  Braid has very little abrasion résistance to rock, but other than that is bullet proof for years.

     I replace mono often, because it is cheap and I am always cutting the last few feet off before I retie. I also change mono every time I decide I want to switch to Floro happens about twice a year.

    I replace Floro every time it gets kinks, over stressed, or runs low on the spool which is often.  I also replace Floro every time I decide to switch back to mono.  Happens about twice a year.

[QED]

I fished 6 lb stren mono that had been on a spool for 30ish years and caught a half dozen bass with no line issues.  The reel was stored indoors in an old black Lew's bag during that time, so no UV or temperature issues.  A quick-and-dirty search shows the consensus view to be that mono only last a few years even when stored properly.

 

So with those prefatory remarks, what failure mode of old monofilament should I have seen and why didn't I experience ANY failure mode at all (OK, the latter question is just rhetorical)?  Bass caught were 1 - 3 lbs; no failures from old 6 lb test...

[Deephaven]

I ruined a spool today, lol.  Skipping docks in boat waves got me.