Could bitcoin "survive" a definitive hard fork, spawning a new currency?
You've described it very accurately. When there's a protocol change that not everyone adopts, you'll have essentially two different currencies, with any coin in the original currency turning into a coin in each of the new block chains. The total value of the coins in both chains will be roughly equal to the total value of the coins in the original currency. The division of value between the two currencies will depend on the respective demand.

However, if both currencies use the name "Bitcoin" to describe themselves, it will be very confusing, and will make both less usable because it's hard to know what someone means when he says "Bitcoin".