One might point out that the Tories had been in decline in Scotland for >25 years before Thatcher, and the Tories had better election results in Scotland in the three elections under her leadership than in any of the five subsequent elections.
A political party that has lost widespread credibility can survive for surprisingly long off the votes of pensioners who have always voted that way; the end only comes when they start to die off. As true of the Scottish Tories as the French Communists...
A fact commonly repeated by internet knowitalls, but one that misses the point. In the 1960s the National Coal Board shut down most older collieries in an attempt to 'rationalise' the industry (the money 'saved' from this was then ploughed into newer pits). The Wilson government's only real involvement was an attempt to soften the blow; it spent large amounts of public money on building advance factories in areas hit by pit closures. Most of these shut in the recession of the early 1980s. In the 1980s, however, the Thatcher government decided to destroy the industry for political reasons and was entirely successful in this endeavour. The Major government finished the job.