I don't understand the attitude that one game suddenly becomes "obsolete" when the next game in a series is released. If a game was worth playing four years ago or ten years ago it's still worth playing today. It doesn't matter whether a new game in the series has been released. I honestly don't understand why a gamer would permanently stop playing any game just because a new one is out now.

To answer the question: I played Morrowind after Oblivion was released and I'll play Morrowind and Oblivion after Skyrim is released. I can't imagine why I wouldn't.

No - the older games won't be "obsolete." Broadly, the only thing that makes a game "obsolete" is when Microsoft changes their operating system to the point that they won't run any more, and that only lasts until it's possible to emulate the old system and run them again. Narrowly, with TES games specifically, I have to be able to play the older games so I can go back and enjoy the good features that Beth cut, and there are always plenty of those.