I'm willing to bet that some posters who swear they would never shop there on principle (or in Duke's case, on principal) would eventually do so, occasionally at least. There are times when you need to buy stuff and you simply haven't got time to drive further away.
I think this is a really interesting and complex issue, and the more I think about it the more uncertain I become as to what the best outcome might be.
On the one hand, there are clear benefits to the community if the college sells the site for a supermarket:
1. The college gets a shiny new building, purpose built and energy-efficient. That is a major boost for the quality of educational facilities for our children and grandchildren.
2. There is an unhealthy monopoly of supermarket provision in Marple. The Co-op has two, and apart from Iceland, that's all there is. This scheme would provide some much-needed competition.
3. Many local residents drive to supermarkets outside Marple to shop, causing traffic congestion elsewhere, and environmental pollution. A supermarket in Hibbert Lane would be within walking distance, or a very short drive or bus ride, for most of us.
And on the other hand, there is one clear disadvantage: an increase in traffic congestion within Marple, and especially around the Hibbert lane/Church Lane and Hibbert Lane/Stockport Road junctions, which are already clogged up at times. Also on Marple Ridge and Windlehurst Road, as people from Disley and High Lane drive to shop here. These are both narrow and winding lanes, unsuited to lots of traffic.
Apart from these factors, there remains one big uncertainty: the effect on local shops. Some posters on this forum seem to assume that the arrival of a Tesco or Sainsbury's in Marple would automatically drive local shops out of business, but I'm not so sure. Ask the people at Hollins Electrical or Archer's or the toy shop what they think of having a supermarket across the road - I bet they don't mind a bit! No, the reality could be more complex - some shops might be affected, and may even close, but others would thrive, and new shops open, as many more people shop in Marple, instead of going to Hazel Grove, Bredbury or Stockport. But it's a free market - ultimately competition drives standards up and prices down: small local shops which continue to provide personal service, and quality at a competitive price, will survive and even thrive.