It's also very important to decrease your expenses during a market crash, and to never panic sell. You could also have some other income streams like rental properties, or software businesses.
If you just want to hold cash and bonds, retire at 30, and expect to live until you're 100, then you need to save up 70 * 100000 = $7 million, and spend $100k per year until you die. (Although $100k in 2089 will be worth a lot less than 2019.)
And who knows, maybe longevity treatments and gene therapies will become widely available in the next 70 years, and people might live until they are 200. Or even indefinitely. If you think that might be a possibility, then you should definitely invest in the stock market instead of keeping everything in cash. There is such a thing as "too little risk" when it comes to investing.
[1] https://www.reddit.com/r/financialindependence/wiki/faq
[2] https://www.reddit.com/r/financialindependence/wiki/faq#wiki...
By spending that 3.5% on living, you are actually losing a portion of your wealth each year.
In order to remain "flat", you need to be earning 7%, in order to spend 3.5%.
...but of course, this also completely ignores the risk. Assets don't give 7% for free. The investor is accepting the risk - that's the whole point. So in reality, you need to be earning closer to 10% in order to mitigate the risk of a default on those assets.
Living off of investment income is almost never really winning battle.