Let's get some facts in here.
The FCC diversion via Tulse Hill adds 15 minutes at most. In peak times it is less, nearer 8 minutes, (exactly as it is today) and that includes plenty of pathing time to fit around other services on an intensely used part of the network. Indeed there are regular examples of late running trains getting from E Croydon to Blackfriars via Tulse Hill more quickly than the quickest schedule via London Bridge. I have missed a good few trains home as a result.
Between 0700 and 0900, ie when the majority of commuters arrive and the trains are busy, there is only one FCC train arriving London Bridge from the south. In the same time, there are 22 Southern arrivals from East Croydon, 8 of which come from Three Bridges or further south. So there is only 1 (8 car) train load of passengers that need to be concerned with the diversion, and perhaps less than half of them will actually get off at London Bridge. For passengers who want the City, there is a handy station called, err, City Thameslink, which is a 10 minute walk from Bank (ie the same as London Bridge). For passengers who want London Bridge, there are plenty of other services to choose from. Alternatively it is a pleasant 15 min stroll from Blackfriars to London Bridge - something I do twice every day.
The alterations from 1 Sept do not result in 'many' services being diverted or terminated short, although that does depend on the definition of many. In the high peak hour today, there are 26 Southern arrivals. From 1/9 it is 23. From 5/1/15 it is 24. Some of the remaining trains will be longer to help compensate.
The principle of publishing the trains in the normal timetable process is simple. It is the standard process, and is well established. In theory there is nothing to stop a train operator completely recasting a timetable - amending every single train significantly - and publishing in normal timescales. Indeed this has happened before, BR were masters at it. To advance the timetable publication would require advancing the timetable process for the whole country, as it is all interlinked. Nevertheless, the service principles have been well trailed for some time, if not the detailed timings (which have only been firm for the last few weeks, indeed Southern are still finalising some details).
