For every dollar (or whatever) of loss for one person there is a dollar (or whatever) of gain for someone else. The lottery everyone loses but the state. Not the same.
The State is also part of the zero-sum game. And you're assuming that Bitcoin is tax free. There's still some uncertainty about which taxes Bitcoin qualifies for. But if Bitcoin ever became a significant currency, you can be sure it would be taxed just as much as the native currency.
Tax evasion might be easer with Bitcoin, but then that's just saving money by being a criminal.
I trade about 10 bitcoins and I'm making good money by buying on bad news and selling on good. It doesn't take anything other than that.
Which is exactly the thing people were saying in every rising stock market. It takes experiencing a crash or two before they realise it's a mirage.
I am not assuming it's tax free. Nothing is tax free. Even if it's not a currency and is considered an asset there would be capital gains....or a capital loss - again you have to look at both sides of this.
Now, with regard to legality, criminality and tax avoidance I ask you to consider if the average taxpayer who fails to report the odd bit of income is more or less immoral than the corporations who shift billions in revenue to tax havens to avoid paying anything. Completely legal, but arguably just as wrong.
I've come through the bitcoin crashes to date and I'm still saying it. Everything since the gold standard went away is a mirage. That dollar (or whatever) in your pocket is a mirage. Value is relative based on perception and bitcoin is as valuable as people think it is, the same as with anything else.