Personally I watched my Grandpa wither away from Parkinsons and eventually Alzheimer's for over a decade, starting only a couple of years after he retired. He spent most of his retired life in hospice care not recognizing his children or grandchildren.

Knowing that is likely my eventual fate too, I really don't want to wait to the last possible minute to retire.

It's a huge shame we can't accurately predict future illnesses and such, we could plan so much more effectively. As it stands we have to do our best and live good lives before we aren't capable of that anymore. Many people won't have a nice retirement. Many won't even live that long in the first place. Society can probably do better for everyone than we currently do