Yeah, they were originally contracted to only do three series. Most British shows only contract for a few seasons at a time, though.
And yeah, Dan Stevens was kinda a nobody and now he'll be know as the guy that left Downton prematurely, lol. I liked Matthew, but there were rumors leading up to the Christmas Special that he was wanting off, so in a way I was prepared for it.
Prior to S3, it was known that at least person was going to die. So when learned the week prior to episode 5 that Sybil was giving birth and then her actress wasn't listed in the cast listing for episode 6 (revealed two weeks prior to its airing) we guessed she was dying.
That softened the blow. I'd say she has a more promising future than Stevens, and she was more gracious about her departure.
I used to hate Thomas and O'Brien, but eventually I came to like Thomas a bit in S3 thanks to his character growth. I agree Rose is pretty annoying. I did like her helping Anna learn the dance for the ball (Anna's my favorite character). I think that they have potential to make Thomas' storyline good in the next season. Rose I'm afraid will seem shoehorned in, but we'll see.