For example, if I make 3 BTC in profit from trading alt coins against BTC in 2017, then does my BTC profit count as a taxable event if I never cashed out?
Any opinions or thoughts on this would be much appreciated.
From this, I think you are not to be made to pay any tax if there were no tax laws at that time because laws are not usually made in retrospect but if at the time maybe there was a bill on the floor and stipulating to capture in retrospect at the document (bill) preamble, then you might have been captured as at the date mentioned in the bill or from the date mentioned.