The thing that will destroy Bitcoins is simple. It will be the bloated size of the blockchain. Soon the blockchain will be so large, that we will need centralized "housing" to hold it. We will have to trust "Central" to state what is in the blockchain every time we want to use Bitcoin. And, this is the thing we are trying to get away from - centralization.
The blockchain is almost 70 gigabytes large right now. Imagine what it would be like if people around the world traded something like the Forex on it

i think before that people are going to get tired of waiting days for transaction confirmations..
as it is now, if i want to sell something for bitcoin, i have to wait HOURS before it confirms.. unless you tack on a stupid high fee.. which nobody does..
i totally agree, whenever i send a transaction it takes around an hour for it to confirm, and it gets tedious when you have to stay in contact with your buyer for an hour to make sure it confirms.