This year it's easy, because I started bitcoin in 2013. I know I made an initial deposit of $X and at the end of the year I had $Y and I subtract them. Otherwise I wouldn't know what the hell to do because I've not only done bitcoin trading but arbitrage, altcoin trading, and altcoin arbitrage, used about 8 exchanges, and half of the exchanges I've used have closed.
Agree, if you started in 2013 and paid one currency, bought Bitcoin, sold them all, got your USD back.
But if you have some Bitcoin left, you need to be calculating their cost basis for next year. If you sold any then bought them back at all, traded alt-coins, it will have changed.