Why, in order to make a "post apocalyptic Oblivion" or a post apocalyptic ActionRPG the fallout franchise need to be butchered?
I think it is premature at this point to say the Bethesda will butcher the Fallout franchise. They purchased the rights to the Fallout IP because it is a recognized and well respected RPG series, with an established mythos and storyline...one that provides the developers with a well defined canvas from which to develop a new game.
Any post apocalypse game is going to borrow heavily from established media on the subject...hell, Fallout borrows heavily from Mad Max for some of its art and character designs...if Bethesda were to create a post-apocalypse America game, it would automatically draw comparisons to Fallout...so why not just make another Fallout game.
Stalker is a nuclear wasteland game, but not really an apocalypse game...and works simply because the developers set the game in Chernobyl.
That being said, I never played through the entirety of either Fallout 1 or Fallout 2...in terms of the isometric RPGs from that time period, I enjoyed the Baldur's gate series and Planescape Torment more...not because Baldur's gate had a superior storyline, but because I enjoyed the gameplay far better...in Planescape's case, the storyline and universe was probably as good if not better than Fallout's, but the pace of the gameplay I found far more engaging.
Quite simply, I wanted to enjoy the Fallout games...but felt the gameplay was lacking...the same can be said for Arcanum...turn based RPGs just don't do it for me...and Black Isle and other developers have proven that you can build very deep RPG experiences without stalling the gameplay with a turn based system.
I am really looking forward to Bethesda's take on Fallout...sure the purists are crying foul, but isometric RPGs have a been there, done that feel to them...an immersive FPS view of the Fallout world could prove rather entertaining and enjoyable.
The Elder Scrolls series, particularly Morrowind and Oblivion, demonstrate that Bethesda is quite capable of developing a rich RPG character creation environment...it seems logical to me that all of the stats and feats from the Fallout games will cross over very nicely into an Oblivion based engine.
Similarly, Morrowind and Oblivion had a very complex and diverse quest based world...the main complaint from both those games is that Bethesda only uses a handful of voice actors for NPCs.
If Bethesda mirrors the depth and wealth of voice actors and dialogue choices from the Fallout series, I think the game will translate just fine.
At the end of the day, Bethesda is developing the next Fallout game...I for one would rather spend my time looking forward to what they create than complain that Fallout 3 will not mirror the antiquated and limited form of gameplay of its first two installments.