I don't understand why you think the slippery slope argument works for the rate and threshold, but not for the existence.

The problem with wealth taxes is not slippery slope, but rather that wealth can be obfuscated and moved around much more easily. Transactions are easy to tax; wealth worth taxing is about what you control, rather than stuff you have.

There's good reasons the most successful wealth taxes are land taxes. You can't easily move land.