Has the Overground been a fair swap?
Not exactly a fair comparison. Back at the end of the last century (1999) to avoid confusion, we had fast trains fro Fores Hill to London Bridge that didn't stop and Honor Oak Park or Brockley. The only problem was that they were absolutely packed at peak time. I would usually stand in the corridor outside the first class compartments or use the luggage carriage. These slam door trains were fast but not particularly comfortable, and they only continued to Charing Cross off peak.
The connections to the south were good with a daily service direct to Brighton. Bot the fastest way to get to the coast but easier. Lots more trains to East Croydon as well as West Croydon/Dorking and Crystal Palace/Victoria. But to the north the East London Line was pointless - very infrequent and not Jubilee line. And when there was a problem on a section of the line everything would collapse like dominos - a problem in East Grinstead would impact connections at Croydon, knocking off the timing for the whole network and amplifying delays through the day. This was the main reason for segregating most of the services, and it generally works - we just need to change more when going longer distances.
Today we have more opportunities for interchange with Jubilee line and London Bridge and Canada Water, improved interchange at Whitechapel including Crossrail, and direct connections to Highbury and Islington. There is also a regular Thameslink service through London Bridge providing additional connections to The City and King's Cross.
Connections to the vast majority of destinations in London are easier and faster, particularly to central, west, north east and east London, and with more alternative routes to almost all locations. The exceptions are East Croydon/Gatwick, charing cross, and London Bridge. And the biggest issues we have at present are Overcrowding on the short Overground trains, and too few services to London Bridge. Hopefully things will improve after December and we will get back to being one of the best connected parts of London that isn't on the official underground system.