Probably the boomers are not liquidating their stock market assets until very last minute - after all it’s pretty liquid so personally I would only sell if I really needed the money or had an a idea of where else to put it for a better short term return. Many boomers also have rental income from properties and other forms of savings.

Finally on average people of that generation tend to save more than they need and still have money leftover when dying.

So based on that I would say don’t expect heavy liquidation from the boomers ever.