That's a pretty good question. While I certainly can't speak for the entire fallout fanbase, my personal option is that if Bethesda simply chose to make a non-canonical spin-off game based on the fallout universe and call it something other than FO3 most people wouldn't even bother protesting. However, if they call their game Fallout 3 people will have certain expectations and if those are not fulfilled then they clearly wouldn't be pleased about it.
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Fanatical Fallout lover here.
It already doesn't meet my expectations. Not 2D like the first two, using some Oblivion 3D engine. None of orignal team is involved. Greats like Chris Avellone, Feargus Urquhart, Tim Cain, Jesse Reynolds, Leonard Boyarsky and many more.
I have my doubts they will be true to Fallout canon and also, that great cheesy Fallout humor as well.