No currency is guaranteed to be deflationary. You may guarantee that the currency supply will behave in a certain way (although even that is debatable), buy you cannot guarantee that the price level in terms of such a currency will continuously fall.
The fixed currency supply guarantees it: unless the economy is shrinking at the same rate, each BTC becomes more valuable over time and that’d cause purchase prices for goods to trend lower.
This has been a key part of the sales pitch for a decade: buy now and it’ll be worth more later, guaranteed, but that’s not good for an economy since it heavily incentivizes holding onto anything you aren’t forced to spend. Since Bitcoin has no innate demand, that’s an especially dangerous cycle since almost everyone has alternatives.
So the only way bitcoin isn't deflationary is if demand for it continually goes down over time until it's irrelevant. If demand either goes up or stays the same, it will behave deflationary (wallets getting lost). This doesn't seem to be a good performance pattern for a currency.
You're right a fixed supply isn't enough to guarantee it, you still need productivity growth and technological progress, something we've had for a long time. Given enough growth a 50% crash is nothing.
