No, I think you are confused.

Bitcoin already exists in the world. People can already buy bitcoin instead of other things, and yet people still buy other things. Why is that?

Nobody says "I'm only getting a appetizer because if I invest the money instead this $20 will be worth more next week."

Investments already exist and yet they don't stop people from spending on other things.

The discussion is not about people randomly buying things higglety-pigglety. The specific point is "Doesn’t this completely undercut the idea of bitcoin as currency?" It does.

Currencies are well-studied economic entities. So are investments. They have different purposes, and people behave differently with respect to them. You can feign ignorance of the two all you like, but that does not erase the difference.

> Nobody says "I'm only getting a appetizer because if I invest the money instead this $20 will be worth more next week."

People in fact do that all the time. People spend less so they can invest more. They trade currency for an asset whose value they expect to appreciate. If that asset does indeed appreciate, they are far more likely to hold onto it than cash, especially if they expect to it to appreciate yet more.

I gave you a specific example of a prominent investor and Bitcoin advocate shifting from treating Bitcoin as a currency to treating it as an investment. Which you ignored, just like you're ignoring basics facts about finance.

The difference is semantic. You don't invest in a depreciating asset (USD). Despite this fact, you still do have and use depreciating assets - cars, inflationary currencies. It just so happens, that your preference is to get rid of bad money first (Gresham's law). When the bad money is gone, all you are left with is the next worst money/ wealth you have (money as in M0,M1 money stock etc.)

The difference is definitely not semantic when we are discussing the utility of a technology for a given use case.

It's like saying all computers are basically the same -- which they are -- and then going on to conclude desktops and smartphones and rack-mounted servers are all interchangeable.

Bitcoin may be a great investment asset. Time will tell. But its deflationary nature and its high volatility make it a bad currency. That's in obvious contrast to the original intention, which was a "purely peer-to-peer version of electronic cash would allow online payments to be sent directly from one party to another without going through a financial institution."

Why is deflation bad?

Deflation is bad for a currency because it means the currency will be worth more later. People hold on to things that they think will be more later. Currency, though, should be something people are comfortable spending.

See here, for example, for a prominent Bitcoin advocate explaining that he no longer uses it as a currency: http://avc.com/2017/08/store-of-value-vs-payment-system/

This is especially a problem when a currency is used by a lot of people. It discourages economic activity and increases the value of debt. Imagine, for example, that you had taken out a Bitcoin-denominated car loan on Jan 1 2016 with a monthly payment of 1 BTC. That's about $440 at the time, or 15 hours at the median US wage. Now you'd have to work 40x as much to cover the loan payment, or about 20 hours a day just for your car. Meaning you would have been driven into bankruptcy.

Luckily for its users, Bitcoin never really took off as a currency, so their debts are in much more stable national currencies. But as you see with Fred Wilson, many experience sharp regret for the times they did use it as a currency.