I used to believe that Bitcoins biggest threat was alt-coins and specifically Ethereum. The BTC ledger does nothing but record transactions (i.e. what happened) but ETH also accepts programming (i.e. if -> then, smart contracts, can issue alt coins to represent tangible assets like houses and cars), which seemed like a significant value-add. Logically too, the first to market tech isn't always the best version of the tech because others stand on their shoulders and build. But so far they're enjoying name recognition status and staying far in the lead - so I hold by my prediction but my timing is clearly too early.