People not spending BTC is what I'm going with.
That's an interesting one. You mean everyone just sitting on their coins, meaning there is no circulation and it ceases to operate as currency, which in turn causes it's value to decrease. Hadn't thought of that one, but it is a definite possibility, thanks.
If its value decreases, everybody would rush to spend them.....
So to bring it all together...
0. new bug found in bitcoin core, which worries people
1. bitcoin value decreases
2. everyone rushes to spend them/convert back to Fiat
3. as prices drop miners profit gone so they abandon processing and shutdown operation
4. network becomes very susceptable to 50.1% attack
5. hacker/gloryhunter who wants to be the one to bring down bitcoin launches an attack (or anyone/any company whose interests are threatented by bitcoin success)
6. alternative cryto PoS or PoW which isn't susceptable to centralisation problems reboots the system
7. alternative is more widely spread in terms of ownership and more secure
8. off we go again