The average market returns referenced do indeed account for downturns, that's why they are "average".

But there is a caveat to that... in order to consistently achieve market average in the long term you need to be heavily invested in aggressive equities. If your position is more conservative, due to either personal philosophy or nearing your desired/mandatory retirement age, you'll miss the big upswings which are *absolutely* vital to achieving long-term performance.

You can also screw yourself if you try to time the market (unless maybe you're Warren Buffett), or go in for fringe, scammy investments.

But if you have 30 years, and invest appropriately for your age you should do just fine, black swans and downturns included. That doesn't include asteroid impact or Global Thermonuclear War, you can always image extreme what-if's but humanity does tend to make an effort to avoid the obvious ones.