Your calculation is based on the percentage will be the same for these big name crypto, which is probably not the case.
I think BTC/ETH percentage cap will continues to drop, so the total cap for other altcoins will see much bigger growth.
If crypto survives and continues to grow, there will be a lot crypto seeing 10x/100x growth, but finding out which ones are among those is a difficult task.
2017
18 billion --> 618 billion = ~34x
2018
618 billion x 34x = ~21 trillion
Can't happen the growth is too high, despite that though we can still have good gains this year.