if the central banks pay more attention to the usage online, there's very small space for crypto currencies to grow.
Very little? The confirmed market for bitcoins are not less than $10 Trillion in value worldwide.
http://www.foreignpolicy.com/articles/2011/10/28/black_market_global_economyAnd this is just the part that the central bankers
can't actually prevent, because it's all already beyond their legal system's reach.
Dude, really. You should try to educate yourself a bit more before walking in to our house and trying to piss on the dinner table. I'm gentle compared to what you're about to experience if you continue to expose an unsupportable position. Most of these guys would throttle you by experience alone.
You seem to be very aggressive, this is not a supportive position for everything in the world. Bitcoin is created with a open/free-will mindset and attitude. You response is just very narrow & aggressive in mind. This is the real world, you can not just live with imagination. Even if the black market/underground economy is really needs the privacy and security, but bitcoin is not the only choice, although it does fit for such transactions but the truth is that people can live without bitcoins because bitcoins have no root in the core economy (like the raw material production, rice, wheat; iron, copper; coal, oil), if can't do transaction like these, bitcoin is just nothing compares to the core economy. I'd like to tell the truth, if everyone like you has no rational attitude, the bitcoin won't grow to a wide acceptable position.
Another truth is that what you said trillion dollars black economy are actually rooted in the USD or other fiat currencies, this is because of these countries corruption or criminal actions, if the society goes for a better level, the need for privacy & security will be lowered. So the need for the privacy & security feature of a currency can grow less OR more if the society grows better OR worse.
And another truth is that Bitcoins are just for personal use, even in the black market, people won't use it to do the transaction like buy oil, weapon, gold, etc. So the underground use of Bitcoin currently is just personally, like the meerly drug deal etc.
If you exaggerate the economy that can fit for the bitcoin, it is possible that you will be put a scammer tag because it's possible you hold a quite large quantity of bitcoins at the early phase or very low price, you are willing to make profit to seduce people to invest in without clear mind of the current true situation.
If you continue to be aggressive, I won't like to talk to you because you are just no helpful to improve anything. OVER.