(Edit: Humans going to the Moon or Mars don't count. People could do that but the distances are significant enough that those are going to be mostly separate economies. They will not impact Earth's economy very much.)
Population growth is the fundamental driver of GDP growth. Growth can still happen in other ways via innovation and efficiency improvements but without population growth it's going to be in the low single digits.
Without significant growth, retirement just can't work. Savings can't compound enough. Taxing the rich won't provide enough and taxing the middle class will kill the economy and make the problem worse.
Lastly retirement is really an institution that was born in an age when people did not live as long (on average) and work was much more physically punishing.
I could certainly see a kind of semi-retirement where a person dips out of high-pressure work in favor of lower pressure intellectual work that can be completed on a looser time frame. A lifetime of experience could probably be leveraged to provide valuable insight or work on high-value niche problems in specific domains, and that can be fairly lucrative and is often very amenable to remote part-time employment.
But regardless of things like that, this is a recipe for pretty significant social conflict. Young people already resent "boomers" for burdening them with cost via not just taxes but squatting on all the real estate and rigging the system to increase its value to fund their retirement. This will get worse, not better.