again, thats only a new issue, with the gamebryo engine. In FO2 places like Reno had probably a few hundred of NPC's in it.

Gamebryo was the main problem with having limited amount of NPC's. The engine can only take so much, heck during the quest where you have to either kill or save Kimball the Frame rate dropped dramatically. It was barely moving.

Fallout 4 will be completely different but thats a different story.

Can you imagine it? many tens of NPCs all engaged in combat or strolling around?

Watch, it'll probably be set on a deserted island.

Yep completely agree with OP.

I've got a moderate computer and been using mods that increase the enemies by about double, it still runs alright so my guess is it's soo barren to mirror the game across all platforms.

Even so, I think they could of at least used the re-spawn tricks that other limited game engines use to simulate vast numbers, Legion could of used waves of attacks in the final fight.

Hundreds of thousands? Are you sure you're talking about Shady Sands and not all of the NCR?

Well, all of the NCR in Fallout 2 which included The Hub, The Boneyard, etc. was 700,000 IIRC. Give a relatively thriving country 40 years and a lot can happen. I wouldn't say Shady Sands has hundreds of thousands, but it is probably substantially bigger and more populated than it was in Fallout 2.

Well sorry, misquote, the wiki entry says it's "tens" of thousands at the time of FONV. The NCR as a whole is definitely "hundreds" of thousands.