I never thought Ted and Robin were all that "awful" as a couple, or that their relationship was "toxic" as others have been saying. They clearly wanted different things when they were younger, and there were some trust issues and moments of jealousy from either side at different points, but it's not like they were throwing furniture at the walls or shooting up heroin together or anything. Lol Ultimately it was always clear that they cared for each other and had feelings for each other. If the Mother had never died it's likely nothing more ever would have been made of it. But she did die, so the fact Ted might try (after a long grieving period) to reconnect with Robin one more time makes perfect sense.