Volatility is destructive to price stabality and general business usage, its a discouragement often stated but also BTC itself has advantages over plain cash and the costs to handling that.   I've always thought Bitcoin will fail if it ever fails to handle the needs of the smallest entities in the economy, whales is a fine thing but they dont support an economy despite their size its often the revenue which matters in a business for economies of scale and other considerations.
   Bitcoin is sustainable is probably most easily considered by how easily anyone can use it for the most normal amounts, I was bearish in late 2017 for this reason but its not been as bad since that peak and some of the backlog was deliberate.