He also seems to think many or most altcoins might possibly decline to near 0 in 2019.
I dare anyone believing altcoins to become near worthless to short them, because instead of altcoins going to zero, your balance will go to zero.
We are dealing with people not caring about fundamentals, decentralization, currency aspect, etc. It all contributes to a situation where even the worst altcoins are worth billions which is pathetic but the damage is done already. It's impossible to bet against altcoins long term speaking because you are really going to burn yourself. Altcoin's get fueled by Bitcoin's increases and as long as Bitcoin keeps increasing, which it keeps doing, altcoins will follow.
Yeah altcoins are not going to disappear, I will agree that many of those coins are not necessary and do have a value of zero but people love to speculate with them because they cannot move the price of bitcoin the way they can move the price of altcoins, so altcoins become a way to earn money and nothing more so it doesn't matter if the coins are useless.