The sad truth is Dan Stevens wanted to leave the show early to pursue other acting ventures. It was the same with Jessica Brown Findlay. Julian himself stated that the characters had to be killed off when the actors chose to leave because both were main characters, central to the family, who had no reason to move away (like Tom and Rose) at the time the actors left.
Right now I'm more disappointed that Gregson had to be killed off the way he did, and part of me is trying to hope that maybe he isn't dead after all since we haven't seen the body like we did for most of the deceased characters in the past.
It was a shame Dan left when Matthew & Mary had FINALLY gotten together & the viewers were just starting to experience their happiness. I think that was part of the reason for such backlash, is that it took forever to get them together & we had no time to really enjoy it.
With that said, I don't see any other way to handle the actor's departure. You couldn't cast someone else. (Dan was too iconic by that point). Nor could they have Matthew always off on a business trip. I think the show really lost something after Dan left. The character of Matthew was such a great personality balance for everyone else.
To be honest and crude, it definitely added to the the drama. I'm not sure what would have come of the storyline had they been allowed to live on as a couple because, as Robert said in the scene when Matthew died, everything with Downton was so perfect: financially secure estate and two heirs - Where would the drama have been to stretch this out a further 3 series?
I think Mary and Matthew's story would either have stagnated with the odd insignificant fight that is expected in marriage OR they may have gone on to show the deterioration of the marriage and divorce because that would allow exploration of society's view towards it (laws, custody, maintenance, how views of divorced men differed from divorced women etc.). I think I'd much prefer remembering Mary and Matthew as a loving couple than a failed experiment.
Plus, Michelle Dockery got to do some wonderful depressed acting.
What do you think?