Something is not making sense to me. You can borrow against assets as collateral and live on the borrowed money, but why??? No bank will loan to you unless you have an income stream from which to pay back the loan. True, the interest would be much less than the tax rate would be were you to sell the same assets. But you must pay the borrowed money back and to do that you will need income which you will be taxed on. If on the other hand you sell assets you will be paying long term capital gains tax which should be lower than your income tax rates.
It's actually two pieces of the tax code paired together that they're taking advantage of. The first is that when you die, the assets in your estate are all stepped up to their value the day you died. Any built-in capital gains are instantly erased. The second is that any money you borrow against an appreciate assets doesn't trigger any tax liability.
So, lets say you have $100M in stocks and they've appreciated significantly so if you sold them you would pay a big chunk of capital gains tax. You figure you're going to need $50M between now and when you die and give what's left to your children and you think you're going to live 25 more years. There are two options. Option one, you sell $2M per year every year for the next 25 years and pay 23.8% in long term gains plus medicare levy tax. So you really need to sell $2,625,000 per year to get $2M in spending money (assuming 100% gains for simplification). Over the 25 years, you end up selling $65,625,000 in stock and leaving your poor heirs a measly $34,375,000.
Or, you borrow the $2M per year you need against your appreciated stocks every year, which as you know literally anyone can do at any broker using margin. If you've got $100M, you're probably borrowing at this point at almost no interest. You get $2M to spend without paying a cent in tax. At the end of 25 years, your heirs pay back the $50M in now outstanding loans and end up with $50M, tax free because of the basis step-up.
Which is why I would advocate that instead of this impossible to enforce valuation of hard to value assets every year, we either tax the loan proceeds of these types of loans for this same group of people, or we get rid of the step-up basis concept. The latter really is indefensible, if you're going to be fair you keep the basis where it's at and keep the taxation system as-is. However that would take a while to realize the revenue, which is where the first option comes in. Either one targets the exploitation of the tax code most folks see as unfair, where as the proposed measure is a blunt instrument purporting to target that but causing all kinds of unforeseen (and in some cases foreseen) consequences.