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.
Gotta disagree with you on that one.
The 618B number you mentioned is the end of 2017 marketcap and the progress from 17B to that marketcap was mostly due to BTC and ETH price rise, not the other, smaller coins.
Look at April 2017 total marketcap chart. That was an excellent time for many cryptocurrencies but since daddy BTC and prince ETH had not started their movement the total marketcap was not much higher than January.
There's still great opportunities in detecting remarkable projects at the beginning of their road and hodling until the right moment, and there will be forever, regardless of the total marketcap.