So I said most of this on /r/academiceconomics but:

• you're way overestimating how much money this would raise even if people paid it. If you try to work out the numbers I think you'll find it's an order of magnitude less than the income tax as it is. Global wealth is about $240T; global income is about $100T in PPP terms. With no implementation issues^lol , you basically get to take 90% of the $240T once (spread out over many years) compared to the third or so of GDP ($30T every year) that your typical government takes in through income taxes, VATs, etc. And if you're exempting, say, the first million or most of the first million from this inheritance tax then that shrinks the applicable global wealth/inheritance tax base dramatically.
• inheritance taxes are usually pretty easy to get around without doing anything overseas.
• there are economists who think very high inheritance taxes would meaningfully hurt the economy, including middle class workers, by disincentivizing saving/investing, increasing costs of capital for firms. More expensive capital means expanding a firm (and hiring more workers to do so) is more costly, "worth it" in fewer cases, and rarer. Hell, something like half of us think the Chamley-Judd result applies to the real world -- that capital taxes should be zero.
• 90% feels super high. 90% of everything or just beyond some high threshold? Are you going to tell middle class farmers and small business owners they can't leave the farm or family business to their kids? That's...Not going to be popular to put it mildly
• a lot of right-leaning people believe the Mankiws of the world that inheritance taxes are harmful, even if they're unlikely to leave a significant bequest themselves. Good luck with your bumper stickers, I guess? You're up against the author of the most widely used Econ 101 textbook, among others.
• what's the incentive for the Cayman Islands to sign your treaty, even if it is popular elsewhere, which it probably won't be?

• if the Cayman islands isn't signing, so there exist countries without this, why should I as, I don't know, Sweden?

• The major issue it would create would be an excessively large state. However, built into the treaty could be vague (as to the particulars) regulations on how the money is to be spent

  I don't think it would create a much larger state because the value of the theoretical tax base is so low, but treaty-based restrictions on how governments can spend new revenue sources are going to be dead on arrival. Especially now, in these anti-globalistic times. What do you think Nigel Farrage or the folks who voted for Brexit would say about signing a treaty that told the UK how it could spend its own tax revenues? The "solution" here is a bigger problem than the problem it's trying to solve

• there has been a constant growth of inequality in capitalism that has only ever been significantly reduced (and immensely so) by world wars. Given that perpetual growth of inequality creates oligarchies and oligarchies make major conflicts more likely, this could prove to be a consistent pattern in capitalism.

  We shouldn't be taking this as given either. I think it even misrepresents Picketty's own claims, but either way it's not a mainstream view at all in economics.

• The policy shift should be stable if income tax is debased following significant accumulation of inheritance tax revenue (no one will back a policy that means they have to start paying large taxes again

  No one ever votes for politicians who want to raise taxes? TIL.