1. They don't sell that much, and more importantly they sell at a far higher price because the market went up. So they comparatively sell more. Studies we have on older people is that they do not actually liquidate theory holdings. What they do is get more loans to invest in rent, backed by their holdings. Or they reduce their money need, by stopping investing in their holdings.

2. Most Boomers are only starting to retire. And they have enormous needs. Plus university endowments, charities funds, etc also are institutional investors. The problem is do they sell. These days usually they only sell to shares buybacks, which actually reinforce the problem.

3. Move pensions to a government tax funded redistributive system, so the income of today pay the pensions of today. Tax land and housing ownership heavily, except for primary house (with a limit based on family size probably for primary house). Heavily (massively. Like 90% at least) tax inheritance. We have good economics support for these, even in heavily capitalistic and neoliberal economic circles. The problem is political. The Boomers will not vote for it, it would take away from them.