People need to think about this for a second. Let's say that you're in Rivet City and you just bought some new stuff that you want to store in your stash in Megaton. Here's what you would need to go through with the current system:
1. You need to leave Rivet City (load screen)
2. fast travel back to Megaton (load screen)
3. go back to your house (load screen)
4. store your stuff
5. leave your house (load screen)
6. travel to the next location.
If there was a Diablo 2 style stash system, this would be what you'd have to go through:
1. You go to your stash in Rivet City
2. store your stuff
3. leave Rivet City (load screen)
4. and travel to your next location.
How is the first way more immersive or "survivaly" than the second way? I would argue that the first way is less immersive since you would need to go through three additional load screens.
Uhhmmm... The first one actually makes sense, where as the second one has you store your stuff in Rivet city.
You said, "Let's say that you're in Rivet City and you just bought some new stuff that you want to store in your stash in Megaton", so you would have to go to Megaton to store it. :shrug:
What you are suggesting is somewhat like going on vacation to Hawaii, buying a bunch of stuff, and expecting to store it at home, from Hawaii, while intending to travel on to Japan.
If there was a wasteland pony express it could seem plausible that your stuff could be sent home (to be delivered when you arrived back).
The game Throne of Darkness is incredibly fun, and is like running seven characters at once in a Japanese themed Diablo 2 ~except not only do three of your PC's stay at home, but they can swap out [instantly] for any other PC's you are traveling with at any time ~its ludicrous... but it gets worse. The game allows you to ~at ANY point (after finding him), access the Blacksmith's shop to buy and repair goods. Meaning that you can be in a cave and drop into the blacksmith's shop to sell what you have found in the cave you are in. I can't abide that kind of strangeness... They didn't even offer a magic rock to take you back to town,

He's just there in his shop ~everywhere you are, at all times.
This stash idea is a no go as far as I am concerned. Fallout 2 had time travel in it, and I'd be better inclined to except
that than a magic steamer trunk with a thousand openings.