It will crash, its only a matter of time. It's also to be seen if it'll crash to a value lower than it currently is. Even if bitcoin becomes a major currency there will be overshoot in its value.
Also for every bitcoin millionaire there's countless people like me who didn't buy and sell at the right time. You see it go up 10%, you have no idea what's going to happen tomorrow, so you say "oh, this is a fine roi." Trusting the market has ruined many more fortunes than it's made and greed is rarely a bearer of good luck.
And finally, trading is like coming up with passwords. You may think you're clever but in reality, your innovative strategy was likely programmed as an algorithm 30 years ago and your perceived edge is likely imaginary. TA bots, arbitrage systems, pump-and-dump commenting botnets, and sentiment analysis of big-data for leading indicators have been used by traders heavily since, well, the beginning - remember where bitcoin came from; everyone who had a coin in 2009, before you ever heard of btc, was likely also savvy enough to leverage all of these.
Also, Do Not Keep Too Much In Exchanges. If history has taught us anything, it's private wallets on private computers that are turned off most of the time is the safest place to store them. If I was a criminal with a 0-day exploit, the very first thing my code would do on a victim's computer is look for wallet.dat.
So have fun, trust nobody, don't panic, and it's probably best not to invest too deep.
The same thing happened when people claimed to be 99.999% sure Trump would never be president, but of course they never actually bet any money on it.
One could argue the grandparent doesn't argue their point either, but they definitely seem to put their money where their mouth is. All these people who are so against crypto and like to sit at the sidelines moaning about a bubble, why don't you either short it or bet against it? A la http://blog.samaltman.com/bubble-talk
It's very hard to predict this market in the long and the short term. As far as I can tell speculation is the only driving factor in bitcoin. Not something I want to bet on or against it's too random for me.
There is an inherent asymmetry between going long and short. If you're going long, your losses are capped at what you put in, and your gains are unlimited. The opposite is true if you are going short: your losses are unlimited, but your gains are limited.
It is definitely more work though and maybe not something most people are willing to do unless they have strong reasons to believe they are right and can profit from it.
(Another way is also to do proxy bets/investments, i.e. go long on things that are inversely correlated with X, but this is less practical in the case of cryptocurrencies for various reasons.)