Amateur Obsessive wrote:
I think the problem is, they're trying to fit a resolution for nearly everyone into the one series. It runs the risk of them not doing any of the storylines justice. Whilst it was nice to see Gwen again, I wish they'd instead have devoted the time to the other characters.
Gwen's appearance was important to the theme of the episode. Tom Branson basically foreshadows it in an early scene when he says something to the effect, "in America a hardworking man can go right to the top in an single lifetime". Then Gwen comes along, and the point is made that she was successful in her own right before meeting John Harding, thus demonstrating the onset of increased social mobility.
This period is defined not only by the decline of the landed gentry, but by the rise of the middle class and breaking down barriers between the classes. A point that would be very difficult to make in a single episode with any of the existing characters,
The reason DA is set in the period it is, by Julian Fellowes' own admission, was because of how dramatically society changed in this period. That is why the first three seasons were set just before the war, during the war, and immediately after the war, respectively.