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."
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.
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.