Great insights, can definitely recommend not taking big equity gambles in your earlier years.
Although perhaps north of $600k is a bit too optimistic, you'd need a solid 6.4% inflation & fee free ROI on that.
In any case it's important to look at it in 'today's dollars'. For example, the S&P 500 did 20x in the past 40 years (roughly the length of a solid full-time professional career), so $10k could have turned into $400k if following that index. But in the 80s inflation went up to 15% causing devaluation, that 1975 dollar is worth about $4.5 today, meaning that $10k turned into perhaps 9x more purchasing power. Of course that's before all the fees get involved. Even if you'd get $600k by your retirement age, it likely will be the equivalent of less than $200k today.
But yeah 6% per year isn't crazy. If you manage to scrape together $50k in your fund by age 25 and work until 65, 500k is very doable, north of 600k perhaps optimistic but who knows what we can expect. Perhaps we'll see gigantic returns once we get 4 billion people on cheap mobile computing, free wifi in the sky, access to modern education, 3d printers, drone deliveries, digital money and a graphene industry haha. But historically speaking $600k is tricky!
Either way, doesn't change the fact this is great advise.