They can pull out their profits at a short period of time but if they think the coin is still doing well, they will still re-invest on it again and again, and maybe they will get to the point of holding the coin for a quiet a long time, only to gain more profits and cut their hassle and expenses for the transaction fees.
We can't predict the future and there are cryptos who do really well for ages but suddenly crash. Their devs decided to pull-off a scam for some reasons. It's hard to believe on it at first but I think if we have benefited greatly at them, we can only move on quickly. They are still better than those early scam projects. It can be hard to detect them but we can start investing small amounts at first.
That's true, they can left the investment at anytime of their choice and if they like to continue too it is their choice. But the risk is that when you have made profit and because the hype of the market you reinvest again and if that your reinvestment time the market collapsed or the coin crashed to the dust then you will be at the losing side of the investment.
Your second paragraph is interesting. Nobody knows the future and seers also made a guest work so in the cryptocurrency market we also predict uncertainty.