Ironic that you would call someone an IYI, yet think that going short is the mirror image of going long. Anyone who has been practicing trading/investing in any form for long enough will tell you that this is very far from the truth (take Buffett as a classic example).
When you go long and you're right you'll eventually win. When you go short and you're right you're actually somewhat likely to lose, still. Best example is all forms of systematic, no-matter-what volatility shorting.
Even if he knew with certainty that BTC would go to zero in, say, 10 years, that doesn't imply that shorting it would be wise - he could easily have his position blown up due to "greater fools" buying in at even higher prices before it tanks. "The market can stay irrational longer than you can stay solvent" and "being early is the same as being wrong" are both well-known maxims in finance for a reason.
Also, if you're wrong, shorts have the effect of not only losing you money, but giving energy to rise when you get liquidated or voluntarily close your position. You must close you shorts at some point, otherwise you go bankrupt, which means buying and driving the price higher.
On the other hand, a long position (non leveraged) on a dying asset is something you ride all the way to zero if your pride demands it, without forcing you to feed into the direction you don't want to be true. So long as it's not 100% of your portfolio, you won't go broke holding it into the ground.
