Grandparents contribute to the survival of their grandchildren. Evolution has selected for long lived grandparents. The same thing happened in elephants and whales.
Their modern “living” in hospices, hospitals, far away neighborhood or vacation resorts have a minor effect on survival on their grand children. Probably negative effect for some pension systems.
Evolution doesn't mean only what is useful stays, it is only when selected for under evolutionary pressure that things are forced to go a certain way, and only after many generations.
The uselessness of old age will only affect evolution if A) there will be a society that is mad enough to really 'do something about it' and B) such a society will eventually outcompete other ways of life and its values start to dominate.
I can imagine a politics where money spend on welfare and healthcare for elderly is minimized to such an extent that life expectancy starts dropping significantly. In a way, the US has something like such a 'make money or die' politics and indeed, life expectancy there is quite a bit lower than would you would expect from such as prosperous nation. Can be economically effective. But it competes which economies where people are productive at an older age, being more focused on staying healthy and significantly less stressed.
Senior citizens today probably have more power (and productivity) than they ever have.
Grandparents used to be younger, too. Women used to have kids before 20, often at 15. You could be a grandparent at 30.
But even old and infirm, still stories could be told, and caring for grandma was distracting for kids too.
There were a lot more kids too. Most women gave birth yearly, and yes more died, but there were still more.
Keeping those kids busy was a plus. Training them vital.
Last 100 years? Forget it.
Human life seems to go through 3-4 phases (roughly youth, middle age and old age). The role of each of those is distinct and fairly clear, but it isn't easy to pick why evolution settled on specific numbers of years.