I think we are getting a bit carried away here. These are major regional centres in their own right, with high rise offices, major shops, restaurants, etc. Orpington station is - well, it's just on a suburban road, Crofton Road. Come out of the station and what do you find - suburban, low density houses. There's a Renault dealer opposite, a kebab shop - that's it. There is a High Street in Orpington but it's a bus ride away from the station, almost more convenient from there to use St Mary Cray station on the Chatham line.
The main reason Orpington seems a major station is it's where the 4 tracks out from London go down to 2, so a place to turn round suburban services. If it wasn't for that, these would be more worthwhile continued through the well built-up 1930s housing area on to Chelsfield, end of London.