We've discussed this idea before.
The biggest reason why I think it's silly (among many reasons) is that the boomers aren't all suddenly magically pulling all their money at once. The generation spans 20+ years, and will keep what money they have invested until they need it. Why would a non-homogeneous group all at once sell all of their funds? It doesn't make any sense.
The best case I think you could make would be a long, steady decline as boomers age and pull out more than they put in. That makes way more sense than a crash. But, guess what? Millennials outnumber boomers! (83.1MM vs 75.4MM)
https://www.census.gov/newsroom/press-releases/2015/cb15-113.htmlSo if you buy into Kiyosaki's "Demographics are Destiny" catchphrase, you should think the market will be going UP, as a greater number of millennials invest than the number of boomers withdraw.
In short, it's basically bunk, and not something to worry about, at all.
Will we have a crash? Certainly, at some point. That's how cyclical markets work.
Will it be due to this reason? No, almost certainly not. Can you predict when it will happen? Again, no.
It's not something to worry about, or buy into.