Fallout 3 and New Vegas are in fact turn-based, but the gameplay system is so robust that you only notice on very rare occasions (such as when you have to wait several seconds after executing an attack to access your PIPBoy in the middle of a fight).
So there wouldn't really be any loss of "turn-based" mechanics, were Bethesda to (VERY hypothetically) convert FO1 and FO2 into first-person gameplay -- it was already present in both versions.
There are things I wouldn't mind saying to you right now, but I'm pretty sure they'd get me banned.
Suffice is to say, no, Fallout 3 and New Vegas are not turn-based in any way whatsoever, and you are completely incorrect for thinking so. The difficulty in bringing up your Pip-Boy is due to animation delay - before putting the stupid gadget up to your face and hitting the "pause time" button, the game waits for any animations your character is performing (usually reloading) before bringing up the menu. This a natural consequence of Bethesda's asinine "let's put cool [censored] in our game without a single thought as to how it would impact gameplay, lore, or narrative" approach to design, an obvious example forcing something that should be easily accessible and functional (a menu) to be of secondary priority to their AWESOME NUKE CATAPULT RELOAD ANIMATION BRO.
What you're doing is effectively looking at what is best a terrible design decision and at worst a bug, and calling it a feature in order to contort into some completely arbitrary and incorrect misconception of what turn-based gameplay is. Which I guess is par for the course. You should give Todd Howard a call, they could use more people like you at Bethesda.