Investing in bitcoin is a risk, as the value can crash tomorrow.
I'm holding too so far, I believe in what bitcoin represents, so I'm taking the risk. Don't invest more than what you can afford to lose however, but if you can, you should buy more.
Also have a look in other cryptocurrencies.
Bitcoin is a good choice for a long term investment. It was already proven by the early investors, but we should always consider that in any investment, there are risk to take. Despite the productivity of bitcoin in almost a decade in market, there is nothing certain in this world, but that is the real thing, if you do not take the risk, you will also end up loosing, so better take the risk, and hope for the best thing to happen next.