It appears the transfer of great northern services are slowly proceeding to tfl grant shapps appears more pragmatic than Chris Grayling. So I wouldn't say it would be ruled out for political reasons.
Any transfer of services to tfl has ot be revenue neutral to the dft and as tfl is under emergency measures currently that's difficult to achieve
The Moorgate Inners are pretty much set for the Overground takeover, with the station refurbishments in the underground section, and the new trains, plus they’re pretty much self contained into one terminal.
The Dartford routes alone are a complex network in their own right, 4 routes (Sidcup, Bexleyheath, Woolwich and Greenwich) into four terminals (Victoria, CX, CS, Blackfriars thanks to Thameslink, and maybe 5 if we include St. Pancras as a terminal), mixture of inner and outer suburban services with mixed stopping patterns.
Im all for a TfL takeover, but I’m not sure if London Overground is the right one to run the SE metro routes, LO is more for simpler routes with one terminal like the Watford line or the Lea Valley lines, and the orbital routes with tube like frequencies.
I think the TfL Rail brand should remain in place for the South Eastern metro.