I was grieving for my "lost" father for a couple of years before he died. Fucking heartbreaking for me, but whoever he'd turned into seemed at peace and satisfied with life for almost all of that time.
I've witnessed the flip side of this: a grandparent with a hard life plagued by worry amplified by the confusion of Alzheimer's. Slowly wasting away for a few years. She did love her sons and grandkids playing piano up till the end, though.
It is truly a beastly disease.
Some things are worse than death.