Was thinking about this the other day. Billionaires and investors have been increasingly converting their stocks into long term assets that have a fixed supply (land and to a lesser degree, houses). You can imagine a scenario where the US begins to look like Europe with our own "landed gentry". So then they would marry sometimes up or down, split land between children, and sometimes sell the land to upstarts when in need of quick cash or payment of debt due to poor management, etc. Or increasingly just not have children at all!! (Also, what is Gates' plan when he is gone? He says he isn't leaving most of his wealth to his children, so why did he suddenly buy 50k acres in Arkansas? Leave to charitable orgs? Cover the state in solar panels?)
Real estate seems like a pretty safe bet until the end of this century when we are predicted to experience population decline. But be careful what kind of farmer u rent ur land to. Many of them are specialists in scamming for crop insurance possibly degrading your land and/or hurting your ROI long term.
(FWIW at least right now I hear the Gates land is being pretty well managed by local folks in Arkansas at least)