Right. So the SNP are not a one-issue party then?
The SNP got 50% of the Scottish popular vote at the general election immediately after losing their referendum 55%-45%.
Clearly - and I mean very clearly - not a one issue party in the eyes of Scots.
The rest, we are both speculating on. You seem to think that if/when they win independence, their 'single' issue will be over and done with and they will fade into obscurity. You might be right but, personally, I doubt that since many scots vote for them despite not even wanting independence.
On top of that, they will be riding the crest of a wave, having already moved on from defeat in 2014 to build support for leaving from 45% to 58%, according to polling in December. Only time will tell.
If you read what I put, I did say that they have widened their policies beyond the one-issue party . Unlike most original one-issue parties they have actually obtained some power, which means they have to have policies on education, health etc. Many voters will support them simply for independence though , as many supported Boris for Brexit. Once they obtain Independence, they will lose their "unique selling point" and simply be judged as another party, though as I have said above, with the benefit in their case of having previous power.