Writing here as the former Chairman of the Sydenham Society, London SE26, which is half inner-city, half suburban, I would say the main problem is the lack of cohesion among local retailers - possibly the lack of cohesion among different groups living in the area. In spoit of having a large Sainsbury’s in the area, our High Street survives, and continues to have independent food shops, with two contrasting ones opening recently. The first is a small Turkish run supermarket, expanding from another nearby centre, the other an up market fishmonger & butcher, run by the son of long established independent greengrocers. We also have a number of successful and useful pound shops - as well as the to-be-expected charity shops, book makers, small branches of a few multiples, and some empty premises. The clientele is a mixture of the less well off, without cars, commuters going into or returning from central London, independent professionals working from home, and parents. As well as Turks, there are sizable Afro-Carribean, Tamil and East European populations in the area.
So economically, it’s not too bad - uses are shifting, e.g. from selling goods to selling time and space to hang out - e.g. by more up-market cafés, but also nail-bars. Bu the changes in demographics and economics mean that the sorts of retailers who used to form the backbone of the local traders association are now a small minority, and reaching the new-comers is difficult.