The history of taxes in the US (and especially California--remember the "temporary" 13.3% highest marginal rate?) has more slippery slopes than 6 flags fiesta Texas. It's not fear mongering, its a very solid argument against new taxes.
My reading of the way the slope slips is that rates go up, not that the minimum threshold is lowered. This reflects historical tax law changes, political sensibilities, and basic economic theory. This was all in my original post as well, so I'm not sure it'll land this time, but I'm a bit offended that you have mischaracterized my post as reflexive, implying thoughtlessness, when I believe it was anything but.
My opinion restated, billionaires have more to fear from this than any of the rest of us do.
Sadly it is legitimate to some extent IMO.
Falsely asserting a slippery slope—that A must lead to B which must lead to C when none of these things are inevitable—is a logical fallacy. It is not a fallacy to point out that the grass is wet and the hill is steep and suggest that perhaps we should put up a sign advising people to stay away from the edge. The argument is simply that this is a dangerous situation which we would do best to avoid, not that anyone who goes near the edge will inevitably lose their footing.
> The billionaires have so much more money than everyone else …
Do they really? It seems to me that what they mainly have is not money but rather illiquid assets, i.e. capital. That includes stocks, which can be sold—at the expense of giving up control of the company—but also stock options which can't be exercised immediately and tons of actual capital equipment, inventory, buildings, and so forth which can't readily be put to other use.
> Income taxes on the rich used to be near 90% in the top bracket, so it's not a one way ratchet.
Keep in mind that they never actually collected anywhere near that much. Those 90% brackets were purely theoretical.
There are many millionaires for whom what you describe is a problem. I assure you that billionaires do not have a problem with liquidity unless they want to.
I think it's arguable that taxes only ever go up. Income taxes on the rich used to be near 90% in the top bracket, so it's not a one way ratchet. That said, I agree government tends to expand and needs to fund that growth somehow. But I think it's much more likely that need manifests as an increase in the wealth tax rate rather than a lowering of the wealth threshold. The billionaires have so much more money than everyone else (and so much more than they need) that it will be much more politically popular to raise the wealth tax on them than it would to expand the pool. Not to mention, there are more of us.
I'm not advocating a wealth tax, and I think it's problematic for other reasons, but I don't think the slippery slope argument is much more than fearmongering in this case.
[0] https://en.m.wikipedia.org/wiki/Slippery_slope