I don't see why they would've chosen to go that way if they hadn't gotten a second season. This isn't Battlestar Galactica
or The Sopranos
or something. They weren't going to give the series a (possible) ending that would leave the kids in the audience sad and hopeless. (Heck, even Galactica
had a moderately upbeat ending.) If they'd known in advance that they'd get a second season, they might well have left Korra without most of her bending as a cliffhanger; but given the need to craft something that could work as a series finale, it's entirely understandable why they didn't go there.
For that matter, they might not have chosen that ending even with the knowledge that they'd get a second season, because we've already seen
a series about an Avatar trying to master each element in turn, and they don't want to repeat themselves. I can't blame them for setting up a second season that lets us, at last, see the ongoing adventures of a fully realized Avatar rather than one in training.