* There are only a limited number of coins out there so people are going to stop mining for them as he near that limit. You can see that now with fewer people buying ASIC and similar FPGA modules.
* The transaction fees are getting be as great as the cost of a pizza transaction
* Lost coins - There are millions of wallets out there with people exploring BitCoin that has just been lost. Its a lot easier to lose your wallet hash then your physical wallet.
Do I think block-chain will take over? Yes and then it will crash spectacularly. I honestly look for the markets to take off over teh hype and then die death crashing into a universal central system.
Look for more slow downs with chat-bots, games, and video services, onion routers piggy backing on the service. Then there purposely bad transactions in DOS terrorism to destroy confidence in issued currency. Eventually this will force the 'new banks' take us back a world centralized bank with sharded nodes that a UN like structure will control.