Agreed. This is something I have been arguing for some months, and I'm pleased to read someone else taking the same view. IMO the main losers, if Asda came to town, would be Boots, Superdrug, Iceland and, as you say, the Co-op. White's, Littlewoods, Neal's, Wilson's can all expect to survive because they offer quality, and often lower prices too (compare fruit and veg prices at Wilson's and Neal's with the Co-op!).
For the rest, most of the shops in and around Market Street sell goods and services that supermarkets don't usually sell - or if they do, it's so tokenistic as to be insignificant. I listed these a few weeks ago, but maybe it bears repeating:
....the hairdressers, the two jewellers, the picture framer, Alan's Paints, Harmony Decor, Crafty Ladies, Sew In, Helen Winterson, Bare Necessities, the opticians, the charity shops, Paul Howard, also Harrods car spares, and the tanning salon and the chippy in Derby Way
These businesses cannot lose - in fact, they can only benefit from the fact that there will be more people shopping in Marple.