Except
a) Blockchain is not a currency but a database. Forced to "replace the unfair banking system" by becoming another baking system, just controlled by teenagers and random Exchanges.
b) This is pure gambling, and that is why its value is not 0. Paypal and hundred of other currencies exist (including "altcoins").
c) It is not a scarce resource, it is not like gold. You can have as many blockchains as you need / want. There are hundreds of Bitcoin clones, some better even.
"It could come down to how much of a legitimate need there is for technologies like bitcoin"At the moment, pretty much none. You can do a similar system with videogame points with a regular centralized database and call it a day. Also, PayPal exists, apps like Revolut or n26 also exist, Moneygram, etc.
And even if Blockchain can be used as a currency and we all agree, there is no reason for it to be Bitcoin itself. As for now, you can see scams, exchange hacks and no user-friendly technology.
Part of me feels compelled to point out growth charts depicting automobiles replacing horse drawn carriages resembled a bubble. The growth of personal computers and operating system manufacturers like microsoft resembled a bubble, as they replaced more traditional paper trail based office systems.
One valid question may be whether crypto currencies represent the next evolution of banking and electronic payment systems. If a case could be made for bitcoin eventually making banking systems and electronic fiat obsolete. What we could be seeing is what happens when someone succeeds in inventing a better mousetrap. In that case, bitcoin wouldn't be a bubble, it might better be described as the future? If that makes sense.
It could come down to how much of a legitimate need there is for technologies like bitcoin. That's something that isn't easy to measure as far as I know. If there's a real and dire need for bitcoin and crypto currencies which will sustain their growth, it may not be a bubble at all.
Also keep in mind the chart which depicts bitcoin's growth could be misleading. Bitcoin reached an all time high of greater than $1,000 around 2014. A case could be made for btc trading at nearer to 10x its ATH, which could be sustainable, rather than the 40 to 60 x some cite back to around 16 months ago.