For example, consider a 60 year old retiree with a 529 plan. Their daughter got into an expensive college but received a merit scholarship, so they only need to use 1/2 of the plan’s balance. The 529 is inheritable by the beneficiary, who will have children going to college in 25 years. The time horizon of the investments in this portfolio just went from near-term/conservative to long-term/risky.
Similarly, there are many boomers who have more money saved up than they could reasonably hope to spend in retirement. They are no longer investing for their future selves, but for their children or grandchildren, who have long time horizons.
Also, most of the LPs in big venture funds are institutional capital (endowments, family offices, etc), not personal capital.
The argument that appetite for more risky investments will dry up (say, decline by 50%) is flawed if it hinges primarily on demographics and interest rates, IMO. These asset classes are far removed from the retirement strategies of everyday people.