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. 

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

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.

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...

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