So you can only have $15mil of assets? But your partner can have $15mil too, as can your kids, and parents. And if that's not enough, you can still try to hoard extra wealth in easier-to-hide forms - whether it's physical gold, bitcoin, art, fine wines, etc.
Or worse still, the super-rich will realise that they can spend many times the wealth cap per year consumable/disposable items (so long as they stay under the cap, or get back there by year-end), and become more wasteful. "Might as well spend a few million on a private jet, if I don't, I'll hit the wealth cap!".