Cosmic
1. Solar Flare causes an EMP that destroys the world's computers.
2. Gamma Burst that kills the human race.

Government
1. All world governments institute the death penalty for mining bitcoins
2. World Law banning excessive energy use for crypto miners.

Tech
1. Quantum computer creates an easy way to decipher anyone private keys from the public address.
2. All ISP ban blockchain nodes as a waste of internet bandwidth, therefore blocking all blockchain updates.

The above would stop bitcoin, although are very unlikely.

Maybe a more realistic question , would be what would stop people from using bitcoin.

Corporate
1.  Banks and Corporations buy up the majority of bitcoins driving the price higher that the majority can afford.
2.  The Bitcoin devs limit the onchain transaction capacity causing the transaction fees to be unaffordable compared to all alternatives such as offchain networks and altcoins. Making sure only the elite rich transact in bitcoin.
3.  Microsoft / Linux companies all have a keylogger / screen logger in their OS that allows them to copy your private keys, and then they steal your bitcoins for their-selves.