If this little survey shows anything, its that most of us don't shop regularly in Marple. So a new supermarket would have little effect on the local shops.
Can't agree with your last point.
I've only been shopping in Marple for the last year but the range of "small" shops is great compared with my previous domicile. Bread bought from the man who bakes it, meat from shops where the butcher knows its provenance, knitting wools, clothes shops which don't stock racks of the same style in the same colour and sell good quality stuff that will last, craft materials from two ladies who can advise you on how to use their stuff, a decorating shop staffed by people who know what a paint brush is for, hairdressers coming out of our ears, beauticians, cake decorators, an underwear shop where the proprietress can sell you a bra that actually fits, cafes that don't belong to chains, a toy shop, a pet shop (again where the proprietors know what they are talking about), proper greengrocers, and a proper stationers, a furniture shop which can also solve your flooring problems, etc., etc. (Apologies if I've left you out.)
You can (and I have done) buy anything in Marple from a needle to a motor car! And on top of all that there is cheap and/or free parking in abundance. I love it.