I have heard lots of hand-waving arguments for why a deflationary currency is bad, but they have never made much sense to me.
A deflationary currency doesn’t mean no one will ever spend or invest. It simply means that there is less forced incentive to spend and consume in order to avoid inflation.
Inflationary currencies are like a tilted playing field, they artificially force savers to spend or invest in risky assets while skewing economic metrics to make the economy look better than it is.
While the gold standard was not deflationary, the relatively fixed supply resulted in strong stable economic growth until countries in Europe began secretly debasing their currencies in the run up to WWI.
Inflating fiat currencies gave the European countries the power to perpetuate the first endless war - I strongly believe if they had been forced to tax their citizens to pay for WWI instead of stealing the wealth of the people by inflation, the World wars would not have been so severe and so prolonged.