Cryptocurrencies aren't necessarily deflationary. Some aspects are quite inflationary, in fact. They can fork. They can have arbitrarily-imposed coin limits increased by changing a single line of code. Other aspects are deflationary, such as the ability for coins to be destroyed or lost forever. How these two forces are balanced against each other is up to the consensus stakeholders (miners) in each case. They should be expected to act in what they perceive to be their own best interests, which could be either deflationary or inflationary, in different scenarios. The interests in PoS and PoW systems could diverge significantly, as in a PoS system the miner's universal interest would presumably be to maximize the value of the currency, whereas in a PoW system the miner's interest would presumably be to maximize both the value of the currency they retained as well as the value of their real-world mining assets.