Actually You count the day it was "mined" versus when you cashed it out. This would then count as A LOSS. Therefor taxes would be low-none. If you never cashed it out, then it's not a taxable event.(They also have no way of telling how much ETH/BTC/ETC your holding). Crypto is only a taxable event when you cash out for FIAT because money for that new lambo had to come from somewhere and I doubt any of us are drug dealers 
Not sure about how the US tax system works, where I live is just like you said, is only taxable if you convert to a government currency. As long as you hold then they can never tax you, and if you buy and hold is the same thing, only taxable when you convert it.