>While the supply will keep growing for quite a while, eventually it will stop growing. And lost coins will ensure that the supply keeps dropping beyond that point. Bitcoin can only avoid being deflationary in the long run if demands drops faster than supply.
That's a very long time in the future. Some coins may be lost, but not enough to have a significant force on the price.
> An inflating currency would never catch on.
>Like US dollars? Or Euro?
US dollars were backed by gold. I'm not sure about Euros but I believe they were probably initially backed in an established currency.
>This is an issue if the inflation is rapid. If the inflation is slow and steady, it's a benefit: It means the value of currency that is inactive - because someone put cash in their mattress for example - steadily diminishes, which gives you a reason to ensure your currency holdings are put to use, whether by spending them, investing them, or lending them to someone who will invest them (e.g. in the form of bank deposits). It helps stimulate economic activity.
People do save money by putting in the bank. If bitcoin got large enough there would likely be bitcoin banks. There is also nothing preventing people from doing this with other resources. E.g. rich people buying oil rights or something to store their wealth in, just like they hypothetically would with deflating cash. Spending money also isn't inherently good for the economy. See the broken window fallacy. Prices are flexible. If someone takes money out of the economy, prices will just go down by that much to make up for it. Reverse with spending.
>As long as the rate of inflation is low enough, people are relatively insulated from the negative effects - you won't really notice price changes from day to day or month to month much -, and the yearly inflation can be accommodated when your salary is adjusted.
Wages are generally the last price to change in response to inflation. Wages are the least liquid market. (Artificial) inflation hurts the general population to benefit whatever group receives the printed money first, before prices rise.