As for Beth using a new engine? They do it every four-five years or so. Fallout used Oblivion's engine, Skyrim uses New Vegas...timeline wise they crop up at about that interval. I think the engine is fine. What they needed was deeper content.
While Mr Howard did overplay the whole "new engine" thing, it's certainly not New Vegas' engine. The Skyrim engine is a major upgrade on the Fallout 3 engine, which was a major upgrade on the Oblivion engine, which was itself a major upgrade on the Morrowind engine. There are huge, huge differences between each literation of the engine. Oblivion added worldspaces, proper AI, formIDs, proper combat, LOD land; Fallout 3 improved on this LOD ten fold, added editor optimisation, improved performance ten fold, added shaders and imagespaces, encounter zones; Skyrim has hugely advanced shaders, depth of field, overhauled dialogue and skills interface, improved disant land, better AI, water, weather etc.
And those are just a few of the major changes for each game. New Vegas' engine wasn't even used by Bethesda, it was literally Fallout 3's engine with crudely and badly added on extras.
Sure, they're all not new engines. But they have enough improvements, especially Oblivion and Skyrim, that I would call them a pretty much overhauled engine from their predecessors.