The old sprites were already made from anno 1997 high-poly 3d models. And the best way to create sceneries is to render them in 3d instead of drawing.
So 3d IS Oldschool an that's why Fallout looks that good.
However, it's really bothersome to convert new renders into sprites, nevermind drawing them. So, for the sake of new content, we must allow 3d to enter the stage.
What bothers me, is the capability of the server to support the data flow caused by 3d. Perhaps i'm in err, and the dataflow doesn't change at all, but people with older hardware could face a problem with their graphics.
Also, current 3d animations suck. They're perhaps good as placeholders. Depending on user's hardware, they don't have outlines or shadows. It's not just a cosmetical problem, but a problem of gameplay as well as it seems. (hiding behind the wall, targeting...). Van Buren critters also look disconcerting to Fallout veteran players, perhaps it would be wise to add new 3d critters instead replace the old ones completely, like ants or rats.
There's of course a possibility to model new critters, which look exactly the same as old sprite ones.