I don't think it was the battle system that made people dislike the game, I think it was more along the lines of poor story and lack of fun minigames in the initial game that doomed the sequels. FFXIII-2 was generally liked by the majority that played it and got much better reviews than the first game.

As for the sales, it's not a surprise, surely? I mean, did anyone actually expect the third game in the series to do better than the second? I think after the second dropped from 1.5m to 500,000 then the third one would be expected to drop down to about 250,000. Maybe that's just me.