Why would anybody prefer a currency that is not stable? Deflation is no better than inflation but for different reasons. It discourages spending and it discourages investment - both of which are necessary to keep an economy functioning.
Do you have an example of any fiat currency that is actually stable? You can't cherry pick what prices to measure either. You have to take a holistic approach and look at asset prices as well as consumer prices. Clearly asset prices in every fiat currency are through the roof, mostly because supply of those currencies are through the roof. A fair, honest price of labor is also necessary - and just as important to keep an economy functioning, and yet workers and savers are getting destroyed. Cities in the US are literally burning, while the investor class is wealthy by virtue of doing nothing, thanks to the glorified counterfeiter that is the Federal Reserve.
I define a "stable" currency as one for which the supply is static - unchanging. That means no one can profit by creating it out of nothing. If it appreciates, it's only because the economy has grown in real terms and produces more for you to buy. If it depreciates, it's due to real recession. Price stability is undesirable in either of these two conditions, and it would require that someone, somewhere, be able to manufacture more currency out of thin air, which is problematic for all the reasons that I've described up to now.
Why would you invest in a business that gives a negative rate of return when you could lose nothing by just hanging on to your deflationary currency?
For the same reason that bitcoin sellers diversify into other assets, or spend their bitcoin. Why would anyone ever spend a single bitcoin, ever? And yet they do. It's the king of deflationary "currencies", while the blockchain keeps growing and growing.
People spend it. For the same reason that people save fiat currencies during inflationary periods. It's not a binary, all-or-nothing proposition. Mild deflation would no more kill the productive economy than a 400% annualized rate of return for bitcoin has killed the bitcoin economy.
Now the real question is, why do you favor the banker and investor classes who manufacture money out of nothing for their own selfish benefit, at the direct expense of savers and workers, out of some misguided desire for price stability, as if? Bitcoin is perhaps the most egregious evidence that we don't have price stability in fiat currencies, at all. Not even close.
