I will bring in a Canadian perspective here. From 1993 to 2011, the Bloc Quebecois (sort of the Quebec equivalent of the SNP in Scotland) kept winning about 50 out of 75 Quebec seats in federal elections (there are always about 20 seats they can never win because they have too many non-Francophones in the them). But by 2011 people in Quebec became susceptible to the argument that electing do-nothing BQ MPs at the federal level was a waste - it was not advancing the cause of Quebec sovereignty and it was also disempowering Quebec in the federal government by having MPs that would be in perpetuel opposition - so in 2011 the BQ crashed from 52 seats to just 4 seats and they did almost as badly in 2015 when Trudeau swept Quebec and people wanted to get rid of the Tories.
I think that's a very good point, but Quebec in 2011 didn't happen in a vacuum. The Liberal Party was also decimated (and tbh, to this day, I'm rather amazed they managed to pull themselves back together). I also remember following that election and I recall that the Conservatives were not expected to win a Majority Government (doing so without even needing Quebec at all). After all, the Conservatives were toppled on a no-confidence vote. I hadn't actually thought about it before, but if the Bloc had held on, it could've been something similar to 1993 except with the Conservatives winning and the left split.
Your point is a good one and should be worse for the SNP. In Canada, the NDP winning the lion's share of the seats in Quebec was a fluke (they'd only ever won a single riding in the province before 2011). I don't think I need to mention what Scotland's political paradigm looked like before 2015. With Labour likely to win for the first time in a long time, I could certainly see Scotland wanting to be a part of government.