When you buy Bitcoin in 2020, you already know what to expect. You know that you will still own some Bitcoin out of every 21 million existing in 2050. If you didn't sell off for fiat. This is not same for paper money.
This is true, but this doesn't guarantees anything practical. There's no guarantee that Bitcoin will have current or higher value in 2050. There's no guarantee that it won't be abandoned for some reason, like having a better competitor or general disappointment in cryptocurrency. If you don't like fiat money because you're afraid it will lose value in the future, than Bitcoin isn't the first answer you should think about, because it can lose value just as easily. Yes, Bitcoin is scarce, but scarcity is useless if demand is low. This is a high risk high reward investment, which is the opposite of a hedge against uncertainty, which is supposed to be low risk low reward.