Vitalik has no idea what he is doing with ethereum. he doesn't even have any faith in its future and he has been clear about it from the start.
When you are starting up something, of course, this is all normal. He was being realistic based on the adoption rate at that time. Ethereum grew massively big in a very short time which am pretty sure he didn't expect to happen given there were already so many other crypto projects in the background.
So, you just can't say he didn't have faith... that's a little "dumb'
example you can use CS:GO game skins and Dota2 items as currency, some of them even worth thousands of dollars! you can go to a restaurant and pay with one of these but that doesn't make them a currency!
I think this is where your problem is... You definitely don't know the proper meaning of currency so let's begin
A currency (from Middle English: curraunt, "in circulation", from Latin: currens, -entis), in the most specific use of the word, refers to money in any form when in use or circulation as a medium of exchange, especially circulating banknotes and coins
From Bernstein, Peter (2008) [1965]. "4–5". A Primer on Money, Banking and Gold (3rd ed.). Hoboken, NJ: WileyThis can also stretch up to
commodity money such as gold, silver etc. If your so-called game skins or Dota2 items however expensive they are, are tradable, have demand and have a circulating supply then hell yeah, they are a currency to the people trading in them. Your thoughts are only narrowed to government-controlled fiat and also having the lame excuse that bitcoin is a currency and ether is not because Bitcoin is a payment coin whilst ether is "a token for smart contract usages."
They all have circulating supplies, are on demand, are tradable and also carry some sort of value so this qualifies them to be currencies (Digital/Virtual/Cryptocurrencies) whether we like it or not.