As a tax lawyer I am more familiar than most people with the details of the tax code, including the estate and gift taxes. Most of the provisions of the estate and gift tax have remained unchanged for decades. In fact the substance of the estate and gift taxes have changed less than the income tax over the last 50 years. What has changed a lot with the estate tax are two things: (1) the tax rates and (2) the size of the unified credit. Much as with income tax, where the rates are tinkered with quite frequently.
I agree with you that the estate tax would be much easier to get rid of than the income, but the reason why is that the estate tax is not a huge source of revenue. In 2016, the last year for which statistics are available, the IRS reported that the federal income tax on indivduals, trusts and estates brought in the most revenue, $1.446 trillion, which was 49.8% of the total revenue collected. Employment taxes (Social Security, Medicare, Unemployment, and railroad retirement) brought in $1.070 trillion, 36.8% of total revenue. Next was corporate income tax, which brought in just $294.3 billion, which was 10.01% of total revenue. Excise taxes, including taxes on alcohol, tobacco, gasoline, etc., brought in $74.5 billion, which was 2.6% of revenue, and estate and gift taxes were dead last, bringing in just $21.3 billion, which was just 0.7% of total revenue.
And it may well get eliminated at some point. After all, it brings in very little of the total revenue anyway. The main reason they don't get rid of it is the politics of it: making it look like the rich would be left off the hook for a ton of tax. No matter that it doesn't really bring in much. Anyway, I took potential outright repeal into account when I said that I expect the unified credit election will remain as long as the estate tax remains in anything like its present form.
If you use the terms a and b trust to refer generically to any arrangement using two trusts, then certainly there are situations in which the use of two trusts may be useful. All I am saying is that if the trusts were set up with the goal of a credit shelter trust, then that arrangement wouldn't be needed today. And without knowing the details of the OP's situation and his/her goals, there is no way I could recommend what kind of estate plan would be best to use. I would guess you would agree with that. I would guess that you would also agree that a trust that has no assets is pointless, and that's what the OP asked about: keeping the b trust but not funding it.