"And we should have culture that would celebrate moving to retirement, to make sure that people do not feel useless all of sudden."
We need something other than just "hanging out".
I am not suggesting that purpose is only found in work, only that getting others to pat you on the back for being retired doesn’t help much either.
Today, somebody retiring at the peak is considered an (economic) loss. Perhaps it shouldn't be.
A non-negligible amount of retired people just sit at home, depressed, until death, out of a lack of better things to do.
Even celebrating their retirement would not make their day less grim.