This developed version of crypto trading itself is quite young, basically since Bittrex and Poloniex exists. From that point, there wasn't a single summer where the total market cap didn't grow. So yeah, that's always. :D

2014 summer: mcap growth
2015 summer: mcap growth
2016 summer: mcap growth
2017 summer: even bigger mcap growth
2018 summer: I've heard a couple of prominent crypto companies taking steps towards helping institutional money to pour into the space. NASDAQ-powered exchange with a little monthly fee + free trading, insane retracement since the last ATH and little upward movement so far. Lots and lots of top-tier companies have started to get involved. This year is the start of the real rat race.

I still believe we get to 5 trillion total mcap at some point this year and there will be alt coins worth the current total marketcap. So ~390 billion / a single altcoin mcap.