Putting what Jeff Booth says (I think) in my own words.

If I save 1000 units of money when I’m 20, that 1000 units of money should buy more stuff when I’m 60, not less.

This is because over that 40 year period all my effort, and the effort of others should have reduced the cost of valuable things.

The cost of valuable things reduces in a free market because entrepreneurs see an opportunity to make money by finding technology to improve quality of the thing, or reduce its cost of production.

In such a world all our lives get better over time because the 1000 units of currency we save when we are 20 goes way further when we are 60. Retiring (or having more time) is enabled by saving early and letting the well directed efforts and energy of humans do its thing.