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