not exactly true.. Todd did answer a question for gametrailers in an interview when they asked "Why skyrim?" and todd said "hes always wanted to make a game in skyrim, with dragons" and hes said before hes always wanted to see dragons in an elder scrolls and is happy to finally do it so it has technically been planned
that isn't planning in the sense that it was being sugested that they had planned all of the other events in the previous games just for this one event in this game. always wanting to have dragons in a TES game set in skyrim is not the same as writting an ultimate plan to end a series. again, key word; wanted. not planned. and nothing in that qoute or any other part suggests that SK will be the end of TES.
EDIT: and todd literally did say something along the lines that the actual conception of what the game was going to be about took place a few months after OB's release. (not exactly they way but it is close)