Depends on the crypto. Fiat tend to inflate since every bow and then they'd pump the economy full of new cash. Fiat these days are basically floating and not backed by anything physical.
I can only talk about bitcoins but so far in the long term it kept its value - so far. Disregarding the volatility of course. I would say it's good store of value but it's OK as an additional store. So fiat savings, cypto and metals.
People who are looking for a good settled future should consider bitcoin as their top savior and helper because only this coin can grant their wishes. Other coins do not hold such abilities. Crypto currencies are going to become medium of exchange in future but at the moment, due to little awareness among people, it would require some item. However, things are getting better for bitcoin and it is the best investment at the moment.
I don't want to see Bitcoin as a savior, there's still many things that can happen but from the view of retaining value, it seem to beat keeping the money in fiat on average. (Again we are valuing the bitcoin in fiat). Among cryptos I'd say BTC is still the most stable one if basing on userbase and acceptance.
I'd rather have some savings in bitcoins in addition to fiat but try to work up to real estate. IMHO nothing beats physical assets.
This is the reputation of a state giving the value to the paper. The stronger the state (its economy being reliable etc.), the better the reputation its currency has. Remove that reputation and like the currency of Zimbabwe - banknote is worth less than a single leaf of a toilet paper.
Well not only that but the government can just declare your money useless, like what India did to its bills. The government can screw your money in various ways.