Deflation increases the value of holding cash. By definition deflation occurs when the price of a basket of goods decreases. Why invest your money in a basket of goods today when it is going to be cheaper to buy that same basket tomorrow?

Let's pretend the deflation is at 3% (i.e. there's a inflation rate of -3%). Then your 100 units will be worth 103 units (in today's money) in 1 years time. This is a guaranteed rate of deflation (bitcoin has a guaranteed rate of deflation, I don't know if it's 3%)

So someone comes along and wants a loan, but they can't pay back 3% interest rate, only 2%. Why would you give that person your 100 units if you'll only get 102 units (in today's money) back in a year? Much better to hold on to it.

So someone comes along and can pay you 5% interest rate. Sounds better, right? However it's for a business. So you don't know if their business will survive, it might go backrupt. Let's pretend it's an early start up, so there's a 50/50 chance they go bust and your 100 units are a write off and gone. So that's only a 2.5% interest rate. Again, why loan it out if you can get a guaranteed 3% by doing nothing?

etc.

Because the money supply of BTC is fixed, BTC are effectively proxies for the value of the entire economy that they're a part of. When the economy grows by 5%, so does the value of your BTC. It's at the absolute optimimum point of the risk/reward curve: you could hypothetically invest in a way that performs better than the average, but when factoring in the risk, your expected value is slightly worse than if you'd simply held on to your money.

With USD, you know the value of your currency will drop due to inflation. In order to keep up with inflation, you have to put your money in an investment vehicle that stands in as a proxy for the overall economy. Hypothetically, you could buy a perfectly proportional amount of every single listed stock, every bond, piece of real estate, and so on (this is what index funds attempt to approximate).

In the latter example, you also have managed to convert your currency into an ideal proxy for the entire economy. Any attempt to pick winners or losers will, statistically, give you slightly worse results on average than had you simply tracked the market as a whole.

Both examples are similar, except in the latter example, you've actually put your money back into the economy: a company issued the stock in order to raise the capital to expand and grow. In the former, your money is essentially removed from the overall money supply. It is available to no-one, and causes further deflation. The more money you hoard, the less money available for financial transactions. The less money available for transactions, the more valuable each unit of currency becomes.

I don't see why any investment would necessarily need to be much better. For one thing, investment is risky, while hoarding money is not.

It increases the return on investment needed for a project to succeed. Simple example: I grab the 100btc and buy machinery to support my new factory. Right after buying, I'm losing money, because machinery won't appreciate like BTC does. So, I'm only interested I machinery investments that return over the BTC deflation rate.

This is a simple example. Some of this already occurs in the real world, since machinery depreciates over time. it's just that currency deflation is another force opposing investment and braking money flow.

Those investments may not exist. Investments can yield a positive return only if somebody else spends money. But what if every potential customer also sits on their money to exploit the deflation instead of spending any?

Risk.