I completely understand, and partially even agree with the "Oblivion with guns" problem classic fans have with Fallout 3, and i completele understand why Bethesda fans might dislike New Vegas too.
Using the kind of argument of some classic fans you could swap the word "Oblivion" with any number of the hundreds of game names produced ... if you overlook that what is unique overall to the game Fallout.
It seems more the fact that Bethesda made both games so, aha, some say "it's just a mod of Oblivion" .. yeah right .. I say play the game in depth and properly and you will only see Fallout.
I assume that when they go back to Oblivion they say to themselves " this is just like Fallout3, but with mediaeval weapons, magic, casting spells, picking flowers and making alchemic potions, becoming wizards, fighting creatures from outer planes, having magic gates opening into those outer planes, which you pass through and fight creatures in those outer planes, and the lush green landscape and rivers, complex dungeons, etceteras " ... "but they both games have inventories, maps and quests" (yeah, and a hell of a lot different) ..... ......