Deflation is a wonderful thing. It means more people can buy more stuff. It should be one of the primary goals of our civilization.
The problem is if you know your money is going up in value, why would you buy stuff with it? You'd hold on to it even harder... thus increasing the deflation, as people lose their jobs, homes, and society falls apart... but hey... your hoard is going up in value.
Deflationary spirals kill people.
It's pointless driving the economy in circles wasting fuel, which is akin to the 'spending is automatically good' mantra. We should be using the car efficiently to get to our destination faster. This means allowing the economy to be more geared towards essentials, which we all must spend on, making them immune from deflationary spirals. And essentials are not only toilet paper, they also include things like medicine and other vital technologies.
When spending power is more concentrated to these economic categories, the competition will increase to produce better versions. Companies will also be incentivised to get a bigger slice of a currency that increases in value over time. Through society's greater savings rates, the banks will have more capital to lend to producers. Innovation, including the move towards (hopefully human-values-aligned) AI and nanotech, will continue because companies without these technologies will eventually vanish.
Finally, if we accept that the environment is dying (or something like that), the real inconvenient truth is because ultimately there are too many consumers. Regardless of how carbon neutral we become, we can never get to zero and no (pre-superintelligent AI) strategy is a match for an exponentially growing population. Therefore, unbridled growth can become a destructive force of its own.