Very good point, marpleexile. If the Co-op let the former Hanbury's shop to, say, Sainsbury's, for a Sainsbury's Local
convenience store, it would probably not threaten the Co-op unduly, because those places are not cheap. But such a move might still reduce the likelihood of a big supermarket getting planning consent.
So by letting to Costa Coffee instead, the Co-op are increasing the likelihood of a supermarket being built on Hibbert Lane, to which they would undoubtedly lose loads of business. However, by doing this they are also (indirectly) contributing to the enhancement of educational facilities in Marple. So good on yer, Co-op!