I hit a milestone last year in which I realized that even if I didn't contribute another dime to my retirement accounts, by the time I was around 55-60 it would to more than fund my current lifestyle. I could spend everything I make for the next 40 years, living life wildly and spending lavishly, and still want for nothing in a traditional retirement. Of course that isn't what I want, so I will continue to save and spend as we currently do.
But your point is different, the idea that I could contribute at my current rate, and even more once the kids are gone, and see how much would be there at 60. I would guess we could hit 8 figures, mostly depending on the rate of growth (since that counts for so much when looking at a 40 year time frame). If growth was slow maybe it would be under 8 figures, but would still be close.
I have no problem with the idea of having that kind of wealth, setting up a foundation that does real and long lasting good in the world would be a fun hobby in retirement. But in reality I won't have that much because I plan on cutting back on work and spending time going on medical trips over the coming years. I only go on a trip or two each year now, and would like to up that to 3 or even 4 trips. That gets expensive, and with the cut back in work the savings will slow down quite a bit. I will still save more than I need over the years, and might retire early if I tire of my job (both employment and volunteer aspects). But I look forward to the security of having more than I need as I get closer to my retirement.