On average it's true. Though it's hard to generate 2MM by 35 (say) with doctor's salary and student loans. Assuming a post-inflation average return of 7%, if you have 1MM at 35 it's only 10MM, but if 2MM at 35, it's 20MM.
Also keep in mind about "working like a dog" for 5 years, basically you are timing the market. There are debates in the investing circles about the true advantage of dollar cost averaging, but from a psychological perspective, dramatic shifts in working/leisure are often damaging to your mental health, especially if you timed the market WRONG.
Secondly, I can't imagine what I would need 20MM for at 65 or 70. Expensive fun things (like a boat or a plane) should be rented and not owned. Old retired people don't need a lot of space. OTOH, in your 30s often you need post-tax dollars to say buy a house or pay for childcare. So if you are generating 750k+ a year post tax, and all of a sudden you need to consume more than the top tax bracket, your marginal tax rate goes way up. It's difficult to plan for things like this easily if your plan is dramatic ups and downs in your income. Especially bad if you have dramatic drop of taxable income, since for a year at least your estimated taxes are way higher.
Another thing that people often don't think about: there's a non-trivial chance of death before 65. On average the chance is about 15% by actuarial tables. Physicians and other high SES individuals prolly have a lower chance, but I'd say it's still significant, likely in the high single digits. There are young attendings who have incurable cancer. It's not as rare as you think. So when you do your cost/benefit analysis you might want to do it with risk adjusted. In fact, as we all know, in the pre-45 group, the biggest reasons of early mortality relate to poor mental health (i.e. suicide, car accidents, substance abuse, etc), and even in the post-55 group, things like poor med adherence, poor diet and exercise and smoking continue to contribute to early mortality. Your 360 degree health is a large portion of your effective total asset, and neglecting that to pursue total revenue generated is not financially sound. I think of this also in terms of asset investment/depreciation. Human capital is a nonlinearly depreciating asset that typically reaches the highest rate of return in your late 40s or even 50s. If you burn yourself out in your late 30s, it's like drinking a design vintage before its optimal maturity date.
That said, in my mind, considering student loans and what not, a typical psychiatrist in a typical part of the country should be financially independent in his/her mid 50s with reasonably good financial planning and without having to "work like a dog" or do "multi-level corporate ownership". Indeed, this is what you see. As I mentioned in a thread a long time ago, senior attendings are in general fairly well off--not like private jet well off, but business class a few times a year shouldn't be an issue.
