Why would the SNP be finished if they lose the vote? Losing the vote would obviously mean that there still existed a reason to go on campaigning for independence. When any other party loses an election or referendum, they don't just scuttle away and say, "Right, that's it, what we Tories/Labourites/Liberals/Democrats/Republicans/Communists/Whatevers want is not wanted here and there is no point in continuing to try to fulfil our aims."
All this nonsense about having endless votes until one gets the 'right' answer is just that...nonsense. Did the Tories go away after the 1997 rout? Sadly, not! Back they came, looking for the 'right' answer for them.
If the SNP win, there is no reason whatsoever why they should change their name just because their primary aim has been achieved. There are all sorts of other 'national' parties around the world.