1.1 What is a currency:
In my opinion just PoW or PoS rewards alone have fundamental economical flaws that make a crypto with just one of them unuseable as currency. A currency is used to transfer wealth and doesn't necessary have to be a good storage of wealth. As long as it has at least some value and doesn't change that too rapidly it's fine to use for exchanging goods and services.
The USD is still the best currency of the world although it's a terrible storage for wealth. Untill 1968 one troy ounce gold was fixed at 35USD. Todays price was at 1300USD so it has lost 97% of it's value vs gold since then. Considering the great ammounts of paper gold now in existance it's right to say that one USD has almost lost all of it's real value in the last 50 years. Interest on saving accounts make this less dramatic, but saving acc are still very bad in comparison with real investing.
No mather what investment asset you could have bought in 1968 nearly all of them had increased dramatically in value by now. Inflationrates on consumer and investment goods are 2 different things. Official rates (even if they wouldn't be faked) are based exlusively on consumtion goods and so have no meaningful message for savers.
1.2 Supply and Demand:
Changes in supply or demand result in changes of the price. Usually increasing prices also increase production since it is more lucrative to produce now for higher prices than it was before. This mechanism stabilizes prices if production can be adjusted fast enough.
Price bubbles and their burst only occur when production can't be adjusted fast enough. Every market has limitations on adjusting production, but in fiat currencies that is virtually instant and virtually unlimited. That's what makes fiat so convenient.
1.3 Supply of Bitcoins (and any other PoW only coin)
Bitcoin mining will always produce the same amount of bitcoins it's supposed to. More problematic than no adjustment on the production it also the decrease of production over time. In fact coins mined on yesterday are just a very small fraction of coins traded today.
For someone to buy bitcoins someone else has to give some away. Fast increasing prices also increase the prices a seller would accept as minimum price. This leads to many price bubbles and bursts and make it very hard to use as currency. If bitcoin wouldn't be used as currency it would be worthless ...
POS cryptos face the same problem here, but this problem goes to extreme for them. Without new coins mined independently of coins someone already has there is no objective method for price disvovery. This makes them by far more volatil so even more useless as currency. PoS is a better storage for wealth since it doesn't burn that much energy like f.e. bitcoin does.
1.4 Why PoS/PoW hybrids are different:
A PoW coin with increasing blockrewards would be ideal for price discovery, but who would buy into such. Only additional PoS rewards could compensate for the loss. Otherwise every new mined coin would reduce the wealth of all coins already in existance and people wouldn't choose such currency.