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.
The blockchain is amazing, bitcoin not so much, at least not as payment or currency. It just doesn't work with such high fees and it's still slow, I can buy stuff with paypal instantly, not with bitcoins. It is the biggest bubble for sure.