What set of conditions (technical or socio-economic) can crash blockchain or bitcoin to zero USD? Is this notion of bitcoin crashing inconceivable at this point 10 years into its existence (so long as there is electricity and the internet)? My corollary question is: Can bitcoin be obsoleted by another cryptocurrency? What would that path of obsoletion look like for bitcoin? For example, I hear about other crypto currencies like Etherium or Lightcoin or Bitcash or Altcoin or Bitgold. What would be signs to look for that would make it prudent to cash out of Bitcoin to buy Etherium, for example.
It's extremely hard to imagine such a scenario.
The worst case scenario would be some sort of catastrophic network failure occurring alongside a complete global ban on BTC. Even then, I think that there would still be a certain degree of demand for BTC when it comes to both sentimental value in terms of collectors (much like the physical coin collectors now), and people who believe that these problems will be addressed in the future.
To address your second question, it could be possible, but extremely unlikely, that any other crypto will overtake BTC in the foreseeable future. Even if one does, it would not render BTC obsolete.