Its a good question, so clever how the protocol works, making it so resistant. I'm yet to see how an attack vector will evolve over time. Layers ontop that become dominant that cause it to breakdown? High fees in the future? Still not clear that anything substantial will happen to cause Bitcoin to fail, that a soft fork can't fix. A hard fork to solve quantum computing issue is probably the most vulnerable issue and this isn't an issue untill many years into the future. The only thing I'm nervous about is protecting your privacy when using bitcoin, but there are steps that can be taken if need to stay completely anonymous.

Bitcoin is pretty solid really.