Cryptocurrencies do nothing to solve the problem.

The fundamental problem is the belief that money is a thing that can be owned and stored when in reality it is just a relationship or a contractual agreement among humans.

The fact that you can own cash and get a guaranteed 0% interest rate even though nobody has signed that contract. E.g. nobody promised that you will get exactly $1 of value in the future. The 2% inflation target is just an expression that this idea is flawed from the start. Think about it this way. The inflation target reduces the effective interest rate that cash holders can demand without anyone agreeing to them receiving 0% interest.

What I am getting at is the fact that everyone, including the economists, assumes that money should be nominally durable. Money is not durable in real terms. It's a market distorting fallacy.

If we assume a barter economy with the option of having money denominated loans but no way to "own" money via cash then interest rates would be dictated by supply and demand and nobody would find it strange that interest rates could be negative when there is an oversupply of grains as the excess grains start spoiling and a negative interest rate could still be more profitable than the spoilage of grains. It's only once you introduce cash, that people perceive the right to pass the cost of spoilage onto everyone else as god given.

Now lets go back to how the modern money system actually works. You go to a bank and you basically promise payment (e.g. through work which ultimately boils down to your time) known as debt to the bank. The bank grants liquid credit which is basically a divisible claim to the debt and thereby a claim to your time. So money is an accounting system for abstract time/labor units.

Well, the unemployed still need to eat and they still need a place to live in. If we are going to be very cold hearted we can call these the storage costs of labor. If you don't pay them, your labor is gone (person is dead). Money lets you pretend that this is someone else's problem, e.g. the government is supposed to take care of the labor (welfare) that you haven't decided to utilize and keep maintaining it.

It's only logical that your money becomes worth less over time (if you assume it is durable). Keeping something in pristine condition requires an external energy input.

Okay, now onto the actual point I am trying to make. The assumption that human needs are endless is wrong, it was only true on a societal level because of endless population growth that is massively slowing doing. Human needs are no longer growing as fast as they used to. There are very few growth opportunities left that let you paper over the inability to express negative interest rates. This also means that the need for investment is going down significantly. Markets are starting to become saturated. Almost nobody but the government is borrowing anymore and this borrowing is meant to raise the interest rate above zero through fiscal stimulus.

If you are a company and you expand production in this environment, then your additional capacity will not earn a profit and even currently unused production capacity is becoming a drag to the economy as it requires staffing and periodic maintenance. Strictly speaking, there is more financial capital in the economy than there are investment opportunities. If saving and investment are balanced, then interest rates must be at 0%. If there is excess saving then the interest rate must go down (even below zero) to encourage people to pay off their debt and get rid of excess capital as the interest on loans slowly approaches 0%.

So there was this final last ditch attempt. Corporations and the rich became net lenders and they simply lent to (risky) households who were buying homes. Bankers noticed some regulatory loopholes and stopped caring about who they are lending to. After all, they are being flooded with money.

A -6% negative interest rate on cash would basically make QE irrelevant and allow price level targeting i.e. perfect price stability with zero inflation.
Because liquid money is now freely circulating there is no risk of deflation which means governments can pay off their debts to reduce the supply of money which stops inflation. Unemployment would be very low as the cost of financing in a saturated market becomes 0% and no longer acts as a profitability gate that keeps people unemployed (e.g. growth dependence is gone). I am optimistic enough to believe that any further productivity growth would simply reduce people's working hours rather than keep them unemployed.

Also, if long term interest rates are near 0% then short term interest rates (e.g. on cash and deposits) must be negative to maintain a steep yield curve. Even if all you believe in is a 0% interest rate on loans you'll still need negative interest on cash to encourage people to lend their money out for a longer duration (certificate of deposit). When you think about it, a bank is using your short term deposits to stay liquid, while writing loans that can last many years. You can, without notice, pull out your deposits and the bank is then basically stuck being insolvent for the duration of those loans. That is what threatened banks in 2008 and this is why we did the bailsouts and are still doing QE. To pretend the damn banks are solvent when they aren't. They are being flooded with an endless amount of short term deposits that can disappear at any time.