He could have a valid point in terms of vertical terminal velocity on charts often being unsustainable in stocks, bonds and precious metals. Of course there are always exceptions to the rule.

The zimbabwe dollars recovery could represent an example of vertical terminal velocity in charts being sustainable:



To answer whether bitcoin is a bubble, someone would need to quantify to what degree people of the world have lost faith in governments and other institutions. How much fear is there that the euro or dollar will crash due to high deficits and overprinting of fiat?

It is possible enough people have lost faith in world leaders to drive concern about crashing dollars/euros/economies to support the growth we're seeing in bitcoin, enough to make it sustainable.

This may seem like a technical issue but it may be fair to say markets are often largely driven by emotion and so the pulse of the crypto community in terms of what drives it, could be a valid point here.