The boomers are retiring, and they're the first major group to retire under 401(k) program. They will be selling stocks and bonds (and whatever else they've invested into) and are no longer buying stocks with their end-of-career top-level paychecks.
Boomer 401ks are probably <5% of the stock market, and they aren't going to 0 overnight.
The idea that boomer 401k withdrawals is going to crash the stock market or tamp out future growth is pretty strange.
Defined benefit & defined contribution pension plans are both larger chunks of the stock market than 401ks.
And all of those together are a smaller chunk than foreigner holdings.
(This is not to say that you, specifically, are a "sucker", and I don't love the term. But I think Taleb has a lot to offer on considering risk.)
401k are a type of defined contribution retirement plan.
As you'd expect the contribution percentage increases with age (and the amount presumably increases even more). But even under 25s are at about 5% which is more than I would have expected though this is presumably just among people who have the option.