1. Strong U.S. or Chinese regulation
2. An irreversible, fatal flaw in the Bitcoin protocol is discovered
3. A competing cryptocurrency, or other emerging form of money, manages to outpace it in growth and steal its thunder
It would recover if it is regulated, but it would take longer. It would be adopted in countries that don't regulate it so much. The chance of a fatal flaw in the Bitcoin protocol is pretty low at this point.
> And I looked at the other cryptocurrencies but I'm not sure they're gaining any traction. If they do, however, why can't they just co-exist with Bitcoin without necessarily harming its value?
I agree, and in this scenario it's likely they would coexist (or even bolster each other's value)
I forgot to mention another possibility, which I'm sure has already happened many times. A large holder of BTC could suddenly crash the market. But the market recovers from this.