Checking the electrolyte is just one aspect of the wet-cell battery. State of charge, that the hygrometer can measure, is just a point measurement - it could be different under a load. Hygrometer cannot measure reserve capacity or load characteristics.
Had one in the Matrix that had all the cells full of electrolyte, SG tested fine. Went out for a road trip - battery cranked beautifully, stopped somewhere to get something to eat.came back out - click, click - dead battery. Just flat out died, fortunately, we were close to a autoparts store, so I was able to buy another one right on the spot to get back home. Battery was almost exactly 6 years old on the nose.
But like they say, YMMV, some batteries last longer than others. I've seen some Subaru Panasonic well cell batteries last up to 12 years! Then I've seen some Delphi batteries in some Toyota die in less than three years. Ambient temperatures, how the battery was charged (overcharged/undercharged), amount of cycling that the battery has seen, etc. all factor in overall lifespan.
For me, it it more worthwhile to replace the battery before it failed, that way I could never get stranded due to a weaken battery. I'm all for replacing it when it dies, but having being burned a couple of times in the past - I just replace them now.