The frugality approach is actually far riskier than many realize. While your expenses may be low now, will they stay low? Will your health take a turn for the worse, leading to huge medical expenses? Will inflation outstrip your income's ability to keep up? If that happens when you're old, you may find yourself in a bad situation with few options.

Personally, I chose to start my own one-person consulting practice, then gradually added a few more micro businesses. These bring in a very nice income.

Then, we balanced our expenses to allow us to set aside a good fraction of that income, investing it in stocks (via mutual funds) and real estate.

The drawback of this approach is that we won't be at FI for a few more years, when I'm in my early 60s. If you want to retire at 40 or 50, you'd need a different, likely riskier approach.