But slower to London Bridge or Cannon Street. Changing at Lewisham for a Southeastern train or Elephant for a rammed Northern Line train is hardly an appealing prospect
More frequency yes, but an Underground train is a lot smaller than a 10 car Southeastern train. And less comfortable too, whether to stand or sit in. By the time an extension happens, the current Networkers will be long gone.
Agreed, for the most part. But, travel patterns will have to adapt, and it's better for the broader network - both NR paths, SE simplification and TfL Tube capacity (Bakerloo and released, e.g. Jubilee at London Bridge). A few stations may see some changes they are unhappy with, but the network, it nets out to a positive.
For example, patterns evolve to availability, especially jobs. Essex and Kent people work a lot more in the City, historically due to the direct links. Essex especially, and Fenchurch folks being a perfect example, with little onward options. Crossrail will massively change that perception and patterns. A role in Paddington is suddenly less of a big deal.
Similarly, Hayes people don't necessarily need Cannon St access (Bromley South manages with Victoria only, and East Croydon have LB only) - and their commuting patterns and appeal of job locations will evolve. They will still have Lewisham for Docklands, and arguably quicker/simpler to the West End.
10 cars @ 4tph is not the same capacity as tube stock @ 24tph+. I don't know what the Beckenham and Lewisham turnback plans would be, but let's assume minimum 12tph to Hayes. No timetables ever needed again - again, an aggregate shorter journey vs average waits or the possibility of missing a train/it being full.