I think maybe you're confusing what you'd like to happen with what is likely to happen (of course sometimes the two do converge). The reality is that while the Scottish Conservatives have a very solid base vote they have almost no capacity for expansion: for Scots who do not already vote Tory (at least some of the time) the party is beyond the pale. Not because it is right wing, but because it is seen as opposing Scottish interests (however defined). I mean it's kind of hard to overlook the fact that the thing that did the most damage to Scottish Labour was the apparently minor transgression of campaigning alongside - and celebrating alongside after the results - the Tories during the Referendum.
Oh I agree Tory support is capped. I just believe it is capped in the mid to low 20s, not in the teens, and that with Scottish Labour incumbents gone, a disorganized party with no prospect of winning, and an actively left-wing SNP which has embraced Labour's traditional Scottish base to the exclusion of older Pre-1997 Tory/SNP supporters, there is potential for it to rise from the 15% or so to 22%.
I mean if we look at the European elections you have the Tories+UKIP getting 28%, which is close to recent polls showing 28% favoring the UK leaving the EU. With no tactical voting, a popular leader, and favorable coverage I think they could make gains. Obviously less likely, and almost certainly less impressive ones than if Miliband was at Number 10 supported by the SNP.