If nobody is willing to sell, then there is no trade. That makes a currency worthless. Bitcoin is even more vulnerable that gold in that regard, because if Bitcoin stops being used as a currency then there will be no incentive for anyone to run the Bitcoin software -- and then Bitcoin will just die.
Collapse will come when no one is willing to buy (e.g. crypto is broken or there is no way to spend/use it). As people anticipate this coming, they'll start selling to anybody who is still willing to buy. The price will drop until no one is willing to buy, at which point the value of commodity will become zero.
On the other hand, if many people are willing to buy (in expectation of future increased usefulness), then the price will go up until no one is willing to sell. At which point every holder anticipates some future benefit, cannot buy more from anybody, but has plenty of people willing to buy from him for some price. This situation will stay like that until anyone decides to "use" money by selling it (e.g. to buy a dinner because people need to eat sometimes). No collapse here.
History has shown that when people start hoarding currency, a new currency arises to replace the one being hoarded. A currency is not useful if it is not being traded -- if the benefit of saving your money outweighs the benefit of whatever you would have purchased with it, you'll save it and find something else to trade. In the case of Bitcoin, there is an even greater risk, which is that people will stop bothering to run the Bitcoin software as fewer and fewer people trade their BTC.
The problem with your argument is that it ignores the utility of the currency. A currency that nobody is willing to trade is not useful, which is harms the demand for that currency. Bitcoin's only utility is as a currency, and so without people willing to trade BTC for something else, Bitcoin is useless, which makes it worthless. Another way to put it is this: your argument only accounts for speculation, which is not the whole story.
History has shown a lot of anti-hoarding propaganda, but not a single empirical example of how it can paradoxically happen that people are buying more and more of some commodity only to have its price drop to zero. This does not sound logical to me.
Your problem is with definition of "trade" and "hoarding". If you rephrase your statements using words "buying" and "selling", then it'll be clear why you create a paradox.
When people "trade" they are doing an exchange: one buys coins for a product, another one buys product for coins. If the demand for coins grows, buyers of coins will compete between each other to offer more of their products for coins. Hence the growth of the coin price in terms of other products.
But the trade (or "hoarding") can only happen if someone is willing to sell, right? So the actual exchange (at any price) will happen only where people are selling coins.
The trade does not happen continually, it is discrete. In the night people sleep and do not trade. And when trade occurs, it is instantaneous. 100% of time money and products simply stay on someone's account.
If at some point everyone expects price to rise and nobody sells, no trade occurs. But still everybody is willing to buy at the last known price. What can we say about price without recent trade? Nothing - we only know last price from yesterday (or last week). Do coins suddenly lose all their value since nobody is trading? No, because people are still willing to buy.
When a single seller expects that the price will not grow further, but there are real products to buy for coins (since everyone is still willing to buy), he will sell his coins and establish a new price. Maybe the same as yesterday, or lower. And the market will find another price. Maybe lower, maybe higher.
Please point out logical problems in my statement before repeating that "no trade means no utility and total collapse".