@Leebo, I had a very similar experience this year.
My first SWG cell lasted almost 10 years which I know is probably above average to begin with. However, my second cell that I just replaced at the beginning of this year only lasted 4.5 years. The difference? I believe it has to do with phosphates but let me explain.
For the first 10 years of my pool experience, I would replace the pool water every year or every other year because of high CH levels. Unbeknownst to me, I was also keeping phosphate levels down as our fill water has over 200 ppb of phosphates. The year after I replaced my first SWG cell, our area came under drought water restrictions and we were asked to not replace pool water so I didn't for the next 4 years. What I noticed as time went on is that it became harder and harder to keep algae at bay with FC levels near the lower end of the SWG recommendations (5%) as I had done for years. Right before the cell failure, I had to run closer to 10%. After the cell failure, I tested the pool for phosphates thinking that it might be related and sure enough the level had climbed to over 4500 pbb. So a few months ago, I removed the phosphates and now am able to run again at a 5% CYA FC level.
So while it may be true that you can operate a pool just fine with high phosphate levels, it does come at a cost of higher FC levels and in my case, more frequent SWG replacement. The cost of those 4 years of cell life far outweighs the cost of the treatment. So in the future, I will be keeping an eye on phosphate levels.