Sounds like you did, in fact, enjoy significant investment gains just by being in the market for a long time. This is by no means guaranteed to be true in the future.
This strategy doesn't depend on amazing returns. If you save 65% of your income, and you currently have $0, you can retire in about 10.5 years (assuming fairly modest 5% return on investments). A 50% savings rate puts you at a 17 year career. It's the traditional "save 10%" retirement plan that really relies so heavily on the market going up long-term. The "live cheaply" strategy is infinitely more resilient.
For context, since 1900, the DOW Jones has returned, on average, 7.5% after inflation. And the S&P500 has returned ~6.5% adjusted for inflation. And in Piketty's Capital in the 21st Century, he shows that throughout history, capital has generally returned around 5% (which, itself is kind of a problem).
But yes. The future is unknowable. There are countless things that could dramatically change these types of financial forecasts.