Money is just a store for wealth, a way to simplify exchange. It has a pretty high degree of utility, which will be hard to get away from. We know that money and wealth are not literally the same things.

The money in my checking account and even index funds isn't "lying idle." The wealth is out in the economy in the form of loans, investments, stock ownership, etc. The 'money,' the numbers with the dollar signs, are just entries in a ledger. They aren't 'things' we can poke with a stick to get to work. The wealth is already out there doing stuff.

A "negative interest rate" is just a tax, an x-percent reduction of the numbers in my ledger so you can fund benefits, government procurement, etc. We know that taxes exist. The question is whether you can tax (or impose a "negative interest rate" if you prefer that terminology) the population enough, at a high enough rate, to fund this thing, without them just voting you out of office.

Because a UBI needs to replace Social Security and all the means-tested programs already out there, but for everyone. The problem isn't that people don't understand the nature of money, but that the numbers don't pan out. "But people don't realize that wealth and money aren't the same things" doesn't address that.