Who ultimately pays a tax burden does not depend on who sends the check to the government; it depends on the relative elasticities of supply and demand. The tax paid by people, wealthy or not, comes either out of consumption or investment. When people consume goods and services, the labor and materials used to produce them cannot be allocated to produce goods and services for other people. The same is not true of investment, which is why you want to tax consumption and not investment.
If the government taxed 90% of Warren Buffett's wealth, Warren Buffett wouldn't consume less, he would just have less investments. The government would have more money, but if it spent the money on giving food to the poor, the food would have to come from somewhere, and it wouldn't be from Warren Buffett whose consumption was already modest. If the tax doesn't reduce your consumption, someone else is paying the tax, not you. Taxing 90% of Warren Buffett's wealth and spending it on welfare programs wouldn't make the poor better off, it would mainly make capital markets less efficient (which would make everyone, including the poor, worse off).
Capital income does make Warren Buffett richer if he makes wise investment decisions that make markets more efficient, but as long as he doesn't consume the gains it changes nothing, and as soon as he or his heirs consume the gains they would be taxed by a consumption tax.
Income taxes have the same issues as wealth taxes, so you want to tax only consumption. And a tax on estate is just an additional tax on future consumption by heirs. It makes no sense to tax future consumption at different rates than current consumption, so it's better to just increase the consumption tax than to add an estate tax.