I don't think a successful 51% attack would kill Bitcoin at this point nor do I think anyone capable would be willing to attempt it.  It would take a great deal of equipment and money to pull off such a thing.  I do like the idea of governments with massive amounts of hashrate closely monitoring chains for any sort of forking while they wait for a massive transaction to receive confirmations.  Makes me think of the new Loki show and divergent timelines being managed by the timekeepers...