Currency failure, have you ever thought of something like that?
Even though I believe the probability of bitcoin's failure as a currency is much lower than I imagined, it should be considered as one of the possible risks that could happen to bitcoin for one reason or another. I don't want bitcoin to fail, but no one can guarantee that bitcoin will survive tens to hundreds of years from now.

To be honest, I never thinks bitcoin would be zero even though bitcoin didn't have any value when Satoshi introduced it. But even then, we just don't need to be so optimistic about bitcoin that you ignore lot of what it is at risk of. Bitcoin is currency that basically still expects more support from the governments of various countries especially in real use cases. If the government agrees to no longer allow bitcoin to be traded, used as legal tender, then the worst risk is losing value.