If SNP lose this referendum then you have to believe that their founding aim and potentially party as a whole is dead for two decades or more.
The uncertainty of Brexit, the suggestion that May will push for a hard brexit against the views of the Scottish people and perhaps the lack of input from outside of Westminster into negotiations all are good reasons for there to be a second referendum. However, if Brexit was a result of Cameron listening to the loudest voices in his party and ultimately the signal of his demise then this may very well be the same for Sturgeon.
And if this is the demise of the SNP what next? The second best placed party is currently the conservative's who care so greatly about the Scottish viewpoint that it was a major weapon against labour in 2015.