I get the point younare making. If the 'one party state' were to be applicable, it would only be so in the sense that alternatives are just so bleak and pointless.
Believe it or not, it isn't as binary as independence and that's all that it is based on.
There are voters who want independence, but don't vote for SNP. They take their chances on voting for parties they align with (huge chubks of islands and highlands where the libdems remain strong for example)
Likewise, there are people not up for independence, who vote for the SNP, because of what they have shown over the last decade in power, and may take their on independence either in another referendum or more open mindedly.
Not huge numbers, granted, but margins that matter. Otherwise the results wouldn't ever really change, and that is why both sides target a wider range.