but they'd have to pay interest on that borrowing, and presumably, these interest from borrowings for daily life and consumption is not tax deductible (or only the portion related to an investment expense could be deducted).
Therefore, to pay that interest, they'd either have to borrow even more, or sell some equity to pay. The current taxation scheme encourages this, because if the person's consumption is lower than their wealth's growth, they would want to reinvest their excess, and this is the most efficient way to do so without the state taking away a large portion from taxation.
Or take a minimal salary or use equity sold at the low long term capital gains tax rate to pay for it.
Also consider that when you have huge amounts of assets - far greater than the loan principal itself - you can negotiate an interest rate that regular people can't. Borrowing 20M for living expenses against assets worth 200M is a very low loan to collateral value ratio.
The very wealthy minimize taxes paid by taking deferred compensation in the form of equity held past the long term capital gains threshold, and then only selling a small portion at the low long term capital gains rate.
The next level tax avoidance approach is as the GP described: don't sell your equity, thereby not realizing any taxable gains, but get cash flow by borrowing against it. This requires you to have so much equity that borrowing a small percentage against it is all you need to pay for your daily life, a position that few people are in.
That's just scratching the surface of tax avoidance mechanisms, but the result is as Warren Buffett described: he pays a lower tax rate than his secretary.