I don't know how you got to the conclusion that the Sidcup line is the 'main line' to Dartford.
The stations on the Bexleyheath line are on average busier. Blackheath, Eltham, Welling & Bexleyheath all see above 2m/3m passengers a year versus only Hither Green, New Eltham, Sidcup.
If anything...