You don't lose until you sell.
Those who bought at $32 were saying the same thing until we crossed it, those who bought at $266 said the same thing until we crossed it, those who bought at $1200 said the same thing until....
"You don't lose until you sell" is wrong. It's called unrealized loss. And actually, this is bad for two reasons: 1) You can't realize a profit, and so your present purchasing power is lower, and 2) You can't even write off the loss on your taxes because it hasn't been realized yet.
Actually this idea of "you don't lose until you sell" was completely true since the birth of bitcoin until all the way to Nov 2013 which was the ATH. Will it still hold true now? Who knows. But if you believe there will be a new ATH in the future, then that statement is still true; it's better to be patient and gain money than to be scared and lose money.
The only problem is, the rise of bitcoin is never guaranteed
No, it still doesn't make it true even if BTC goes up in price. If BTC goes up and he sells, he realizes profits. Losing additional profits isn't a loss at all (unless you're a glass half-empty kind of person).
I'm guessing that you meant something more along the lines of "you don't profit until you sell" when the price of BTC goes up. But, in this case too, you profit in terms of unrealized gains.