I'm always on the pessimistic and risk-averse side with rising asset prices - I still believe that cash in your hand is king, despite all the woo and yay, whether it's cryptocurrency or real estate. And I'm frequently wrong - some things do seem to just keep growing in value.
But man, Ethereum with a market cap of $11bn going on $12bn? That's not far short of Twitter's $13bn and Twitter's been going more than ten years. When I went to see the Ethereum London office a couple of years ago, it was like three of them and half of them seemed insane. And BTC pushing $2k, so the average house price in this country is just over 100BTC - surely, something's got to give.
why does something 'have to give'?
because sooner or later, people will see more value in the prices decreasing than in having them increase, or something will go badly wrong - see how Ethereum took a dip when TheDAO got hacked for example.
But mostly this isn't about reason it's about the gut feeling that these things have got too big, like a balloon that's ready to pop.
You could say the same thing about real estate markets around the world, and stock markets around the world, when they go pop where does that money go? Cryto's are now seen as a safe haven asset