An outsider's perspective -
The finale was the only episode of How I Met Your Mother I ever watched. My girlfriends a fan, I just never got around to it.
I think if your ultimate goal is to send your fan base off with a happy ending that it's a pretty terrible idea to paint the Robin character as pretty much the coldest, meanest person on the planet for 55 minutes and then end it with 'Hey! These two crazy kids are getting together!'
Eh, I think you're pretty far off from what they were "painting" Robin as.
I don't think it was really their goal to send off the fan base with a "happy ending," per se, at least not the storybook ending many fans seemed to have wanted. I think they gave the fans a bittersweet, fairly adult ending. I appreciate that.
Robin was a career-focused woman who didn't want kids, and then later found out she couldn't have kids even if she wanted to. She and Ted weren't quite right for each other throughout most of the series, and Robin ended up trying a marriage with Barney, and it didn't work out. Barney was inspired by Robin to try and change his ways, and he succeeded for a while. Then their marriage fell apart and he reverted to his old ways, until having a child by accident changed him. Robin was sad about drifting apart from the group and having her marriage fail, but she poured herself into her work. Ted got "The Mother" and he had the whole "married with kids" dream life until misfortune cut in and took his wife away from him too soon. Then, many years later, suddenly it makes sense for Ted and Robin to give it another try.
The show was called "How I Met Your Mother," but as the kids said, the story was really about Robin the whole time. In telling the story, Ted was in a way convincing himself that it was okay to move on. Ted was an unreliable narrator for the entire length of the show. I liked that. Ultimately we saw that after The Mother's death, for a while her greatest fears about Ted were confirmed -- he was a guy living in old stories. But finally at the end, with the help of his kids and the growth he experienced with The Mother, Ted is able to move on, even though he's "going back" to Robin.