This is getting interesting, this discussion regarding risk is very tough because everyone defines risk differently.
If you are an emotionless robot, then maybe boarder's 3 risks are accurate. I would also add #4 into that list of risks losing your job and being forced to live off of depressed assets for a prolonged period of time.
Me personally I would rather work 1 extra year now for example at my inflated wages then have to work at McDonalds after retiring. I am very comfortable mitigating that risk with an extra year of work. Especially since my work is not unbearable.
Then there are other risks which are much more difficult to quantify. For example the risk of stress and being able to not sleep well causing you health issues because the market becomes too volatile. This is a real risk us non robots have to deal with. In all honesty health is one of my biggest priorities. If I can use money to purchase better health with very little effort, then I think this is a well placed purchase. So how much could health cost me? Is it that same 1 extra year of work I described above? If so, then again, that is money very well placed and I am thrilled to lose a few percent off my returns to be able to fund my health.
You can't really understand your own emotions until you have experienced a significant downturn when all the news and the people you meet are spreading doom and gloom. Especially when your own portfolio drops by several hundreds of thousands. You may be aware that at some point in the future the markets will rise, at the same time the lose is staring you in your face. Every month you are selling stocks that have lost a large chunk of their value and your wealth continues to decline. That is some scary shit, and I am confident that my health will be affected if I am not properly prepared. BTW, working at McDonalds will not decrease my stress. It may diminish it a little, but after a month or two flipping burgers I think real doubt would start setting in. Instead what I choose to do is make sure I have the right asset allocation. I mitigate my risk by making sure I have enough bonds or CDs in my portfolio to survive several years without touching my equities. Maybe even selling some bonds to balance back into those equities to make some significant profit.
When that next recession comes, and it will come, I don't want to struggle, I don't want to worry, I just want to keep on trucking and enjoying my life. I sure as hell don't want to be stuck in a $7.50 and hour job when instead I can have a pleasant vacation in Tuscany. I am very happy to spend an extra year working to secure that retirement with an appropriate asset allocation that meets my willingness, ability, and need to take risk.