I agree that they need to rent it out. I know the nearby shops don't want a supermarket, and really understand that, but maybe it would be better than a superstore on Hibbert Lane? At some point there needs to be competition for the Co-op, even many NO people agree.
Shopping habbits have changed and the shops doing well are the bargain type of stores like Poundland, B&M/Aldi etc. Any of those would threaten current businesses also, it is unlikely to stay the same for years and years; nothing does. If you look at it another way, though, if someone doesn't have to do the trip to Stockport for certain things, they may spend more in Marple in other local shops. I know that when I have to go into Stockport, I often go into other stores. If I can get what I want in Marple, I prefer not to go any further and often have a nosey in other shops and end up buying from a few shops (but many things you can't get here-kids shoes etc).