An Aldi or Lidl WILL create and give much needed;
1) Jobs
2) Choice
3 Locality
4) Much better use of the space
5) Improved services
6) Better value for money and a more diverse choice than is currently available
7) Much needed competition.
I think points 2, 5, 6, and 7 are essentially the same point made with different words.
What would give consumers more choice would be an Aldi on Sydenham Road (where Budgens used to be) and another in Bell Green where Toys R Us was located. Opening two stores would double the number of consumers who would benefit from the choice and competition provided. It might also provide more jobs than a single larger store and two empty units.
In terms of 'better use of the space', I believe there are better uses of the space than another warehouse supermarket which threatens the viability of town centres. One of the problems with 'choice' is that large companies put smaller shops out of business and actually reduce choice. That's why there isn't a fishmonger or greengrocer in Forest Hill - because Sainsbury's have taken over all that business in the last 30 years.
But at least Sainsbury's is located in the town centre, and that assists other local businesses. It is the reason that The Butchery is able to thrive directly opposite Sainsbury's. If we want a diverse high street we should encourage major retailers to set up there, rather than locations away from public transport and normal high streets.
All the points you make are negative points for our town centre: Less choice, under-utilised space, and loss of locality. At the same time it fails to address the thing that we need more than duplication of supermarkets, and that is housing.
I hope you'll appreciate that my perspective isn't primarily a NIMBY argument, it is YIMBY. Yes to housing in Bell Green, and yes to Aldi in the town centre.