I agree there is legal risk for people that fail to report capital gains from bitcoins that have appreciated in value, relative to fiat, once they convert them back into fiat or tangible goods. The IRS, or its equivalent in other countries, has a long arm.
But declaring the capital gains rate 100% with respect to a specific asset? I think that would require a change in the law and I don't see it happening. I don't think capital gains incurred from even outright illegal activities are taxed at a different rate, are they?
No, they probably aren't, but penalties are massive. Anywhere from probably 20-40%. And those are just penalties. I wouldn't be surprised if there were other possible fines on top of that.
If that guy who bought that Porsche doesn't report it in some form, and then gets caught, I bet he'd owe more than the value of the car in penalties and fines sooner or later. One can't claim ignorance with these things.