That's because both Barney and Robin can do stuff without it affecting something that's already been established. With Ted unless it's the mother we already know the relationship is going to fail and that's really all Ted does.
And the Office went bad before Steve Carell left, it's just he was really the only good thing left at the end of his run.
Yeah, I personally say that it was a bad move saying that robin isn't the mother so early in the series.
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