You're missing something there, is it the price? It should be: BTC will be over 10k USD by 2020. That would really be a disaster when compared to the current uptrend.

In reality the worst scenario would be if somebody managed to crack SHA 256 or hack Bitcoin some other way and make people lose confidence. We know that the governments will never work together to ban it and it won't die by itself. The only way for it to happen would be if people lost confidence and abandoned it.

The best scenario would be total adoption by a number of countries that would allow it to compete with national currencies.