Who said anything about Sturgeon 'fading away'?
Your point was that the way things are, she could well be in position as a 'quasi-dictator' for the next 30 years due to nationalist sentiment in Scotland.
If Scotland was independent, every single person who is drawn to the SNP purely as a means to an end, i.e. Independence, suddenly no longer has this as a primary motivating concern. Would Nicola Sturgeon still remain in place as SNP leader? Who knows, it may be that she would feel she's done her bit and stand aside, but even if she remained in post, it would be as a leader of a party that is suddenly shorn of all the voters, members, activists, and politicians who only ever regarded the SNP as a vehicle to achieve Indi.
I am not a natural SNP voter myself. I never voted SNP in my life prior to 2007. I would seriously consider the policies of any left of centre or moderate party that should exist in Scotland once Scotland was Independent. I'm far from alone in this view, and while I believe the SNP likely would persist as a political entity post-Indi, I don't believe it would resemble anything like the behemoth it has become over the past 15 years or so.
What's the alternative in the meantime? That Scotland bumbles along with upwards of 50% of the electorate united behind the SNP, and the remainder split between the various pro-union parties, parties which will remain completely unelectable because they can never win the votes of more than 20-30% of the electorate?
It's not healthy, but it's not going to resolve itself by being 'more union' and ignoring the will of over 50% of the electorate. That's what's driving nationalism more than anything else. The unionists are painting themselves into an ever shrinking corner because of their pig-headed refusal to acknowledge that the constitutional debate is of crucial, defining importance to at least half the Scots electorate. Get it out of the way, clear the decks, then we can start with a clean slate.