Yes, holding Bitcoin for the long-term can be a good investment strategy for those who believe in the long-term potential of the cryptocurrency. It is true that the price of Bitcoin can be volatile in the short-term, but over the long-term, Bitcoin has shown a tendency to increase in value.
If someone is still in doubt about that fact, they can hover over the chart of Bitcoin and look at its history.
That's more than enough to prove that bitcoin is something else and they can argue anytime they want if they're doubting it. But they'll never change the proof that we've got mostly as we've experienced the essence of having bitcoin. Having it is one of the best investments that one can ever have.
Those that keeps on doubting it will just see the other holders profiting from it at the right time when the bull run arrives.