> Does investment really grind to a halt though?
Well, it's discouraged, but yes, basically. Under inflation, you need to invest money just to stay even. The lower inflation is, the less urgent the need to invest. With deflation, you're somewhat incentivized not to lend. A fixed money supply guarantees deflation, so yes, this would significantly reduce lending and the economic growth that lending stimulates.
> Also, inflation means everyone needs to be investors or the money they worked hard for evaporates. Why should we force Joe Sixpack into being an investor for the good of the economy?
If Joe Sixpack lives paycheck-to-paycheck he can ignore inflation entirely, it doesn't affect him a whit. Once he gets some savings, if he doesn't like risk he can buy bonds or something and continue ignoring inflation. But since you brought it up and since this seems to be a point of confusion for a lot of people, yes, let's see how a fixed money supply would affect Joe Sixpack.
If the government stops printing money, we are guaranteed to get deflation any time the GDP grows, right? With money supply held constant, "The US currency deflated by 2% last year" is equivalent to saying "The US GDP grew by 2% last year." And what makes the GDP grow? Joe Sixpack and his friends, out there mining ore or building furniture or otherwise increasing the amount of value in the world.
2% deflation also means that anyone with savings sees the buying power of their savings increase by 2%. If Scrooge McDuck has $5M in his mattress, his inflation-adjusted net worth increased by $100,000. He can retire and spend the rest of his life living very comfortably on the equivalent of $90k/year, and despite him doing no work, despite him incurring no financial risk, somehow every year he'll get a little wealthier.
How is that possible? Who's paying for his life of luxury? Joe Sixpack. Deflation is quite literally a tax by capital on labor. As long as the GDP is increasing, arguing for deflation is tantamount to saying, "I think some of that GDP growth created by workers should go to anyone with money." AFAIK this is universally understood to be a Bad Thing (tm) by virtually everyone.