I have long written that diversification is a very smart idea. No one can predict the future, etc. But discussed below is another reason not be 100% in ANY investment, at least for long. I ran into this argument at a gold blog.
Imagine that you are 100% invested, "All Inn", in gold. Even if the price of gold were to go way up, there is still a big risk that many don't see. Namely what happens if there is a big price drop JUST when the owner might NEED to sell (eg, an unexpected emergency). If our imaginary friend bought in at $1275 gold (approx. price today), and then price drops to $900 (Martin Armstrong predicts a sharp price drop like this, prior to a big price rise, a "slingshot" price rise after its initial drop).
And then, just at a bad time for the gold owner, he might need money (US dollars) to cover an unexpected $200,000 medical bill. And he if forced to sell his gold at a 25% loss to cover his bills... Ouch! It would hurt even more should gold then go to $2500 per ounce.
So, it is unwise to be All Inn on gold, even if we were to be very sure that $2500 gold is coming.
The above scenario would hold for Bitcoin as well, or anything else to be held long-term.
Some of us gold owners have a saying: "Protect the precious." That means keep some powder dry (CA$H on hand) for the unexpected.
Still, in my country the health services are in the public sector, and we have safety nets in some cases. If you're wealthy though and you pay for things often it's always a huge problem. People ignore that the end goal of this isn't money, it's enjoyment - you're not just saving until you're 100 years old and your back hurts too much to go paragliding.