Obviously the main platform for SNP is to obtain independence but there is no reason why the party itself (like others in the past all over the world have done) can't adapt its raison d'etre to be running the country after independence is gained, this whole idea of it not existing because its objective has been achieved is flawed.
And that is the concern as most politicians are much the same, irrespective of the party they represent.
ETA Would this new party be any better or worse, that's the big question that nobody can answer with any authority.
Given the country's political history, a proper Labour party. Separate and different from the sham UK one.
ETA
At the moment the two major UK parties are Tory and Toryer.