I have 2 main fears...

1.- Cryptos getting banned all around the world... Governments could easily block the ports that use cryptos and make from it something illegal with a lot of years in jail if someone uses them, that would be the end.

2.- Crytpos getting nuked by quantic computers... If the most powerfull quantum computer generate all posibles private keys, then he could transferr al the bitcoins in one single transaction.