That's fine for you, but it's not fine for the vast majority of people.  Diversification is key, not matter what silly statements anyone makes up around it.  Only "investing" in bitcoin is foolish and short sided.  There's a big difference between cryptocurrencies and other types of investments such as stocks, which are publicly trades shares of a company with an actual product or service, which is very different from bitcoin/crypto.

I would recommend no one hold all of their eggs in one basket, especially when that basket is a highly volatile currency in it's infancy.

I have done a write up about this very topic here- https://bitcointalk.org/index.php?topic=5288768.msg55577767#msg55577767