Given the factor that only a small percentage of people knows cryptomarket, there is really a huge chance that it will grow to what OP is thinking, and further more, I believe it will even surpass this market capitalization. Once country are able to regulate cryptocurrency and had adjusted its banking sytem / finance system to the trend of cryptocurrency, they will adopt it one by one thus creating more market for cryptocurrency. with this factor alone (government accepting and regulating cryptocurrency) will definitely boost the market cap though, I am a bit hesitant if it will happen this 2019.
But 6 trillion is quite high, of course we could reach that market capitalization but I will prefer that to be stable growth instead of reaching that market cap and then lose 70% in the next months, also I do not think governments are too eager to adapt to cryptocurrencies, the bigger is the market the bigger the menace to them and the greater the regulations they will want to create in the market.
This is more than impossible sort of thing to happen in this much short interval of time. The market is growing at a faster rate but still, reaching this much value is very much difficult to achieve. We can really hope that the worth of the crypto market will be much higher by the end of next year and that by that time, lot of other people would also have invested their money into it.