Learn about time preference. It's one of the most important and relevant concepts in the Austrian theory. There is no such thing as money that just sits there forever. Saving money means having a low time preference for current consumption, which means delaying the consumption for future needs. For situations where the time preference for consumption has increased. Therefore saving, or "hoarding" as some idiots call it, is not putting money away for good. It's saving money for future consumption.
If you have so much money that you could indulge all of your present wants and needs and still have money left over, then effectively, wouldn't the remainder "just sit there forever?" Is it really worth entertaining the notion that the money will enter the economy in 100 years so it's still productive? What about 200 years? A billion years? It sounds ludicrous but if you had a certain quantity of money that you didn't need to spend, and it continued to appreciate just by your holding onto it, then wouldn't you just hold it, and if so, how is that productive?
It is productive in that it keeps the economy solid and honest.
For example, it can be used to steer the interest rates. If the rate of interest rises high enough, the owner of the megastash can start lending them out to the market. This gives the new entrepreneurs more cheap loan-capital that would not be there otherwise. This facilitates growth of the real economy. Also the contrary is true - if the owner has already maxed out lending, and the rates keep on going down, he can smell rotten and decide to withdraw his lending from the market, and put it in a cold wallet instead. (The "rotten" he smells, might be for example fractional reserve lending that feeds the market with illusion that credit is available although in fact it isn't). This way he cools down the bubblish economy.
If you consider what is happening in the fiat world, we are in the very final stage of the credit bubble illusion. The interest rates are the lowest in world history, still nobody wants to borrow (I mean nobody with the actual ability to pay back the loan). Only governments and individuals who are broke, do borrow these days. Wealthy people and productive businesses either avoid borrowing like a plague, or are disqualified.
I own more income generating businesses, gold, silver, 500 euro notes, and bitcoins than most of you. I asked for a 7-year loan to buy a EUR 420k house, I would have put EUR 120k down and the rest in monthly installments that were within my family's monthly net income. The bank did not even bother answering me! Also my friends report about similar things.
So I smell rotten here. I won't keep my money in the bank earning exactly 0.0% interest if they are not doing what they should, loaning it out to the most creditworthy borrowers imaginable. If they keep on buying treasury bonds instead, it tells in plain letters about their appetite for ponzi and scorn for the productive economy.
Buying bitcoins now, for some it is a way to riches. For others, it is the ultimate manifestation of distrust in the banks.