Iirc, MMM's savings were buoyed significantly by being a part of the boom years of both the stock market and real estate market. Whether his financial standing can be replicated by someone following his exact footsteps today remains to be seen.
The returns on your investments may certainly vary under lots of circumstances. But if you're able to living on under 50% of your income, then we're simply talking about how good the outcome will be — not whether it'll be good or not.