I think the SNP has reached a peak in its popularity, and the recent election has now shown this to be the case, it couldn’t form a majority in Parliament (I’m aware it’s designed to prevent a overwhelming majority government) it lost the referendum, and the Tories have become the main opposition for the first time since the 1950’s.
The SNP has also got a growing problem with an increasing budget deficit, which is projected to be three times greater than the rest of the UK’s. I’m not predicting it will be defeated soon, it might take 10 years, but it’s not going to see the levels of support it has seen in the last 2 years.