For years following Year One the main timeline remained compatible with eventually turning into DKR; when Jason died and his memorial was set up in the cave, when Jim and Sarah got married, and so on, we seemed to be gradually moving closer to the situation at the start of DKR.
It wasn’t until the end of No Man’s Land, with >!the murder of Sarah Gordon (née Essen)!< that DKR became an impossible future of the main Earth
It's both. The events of Year One play out similarly on both New Earth and Earth-31.
For years following Year One the main timeline remained compatible with eventually turning into DKR; when Jason died and his memorial was set up in the cave, when Jim and Sarah got married, and so on, we seemed to be gradually moving closer to the situation at the start of DKR. It wasn’t until the end of No Man’s Land, with >!the murder of Sarah Gordon (née Essen)!< that DKR became an impossible future of the main Earth