Ok, let's look at some other games from 2008 and see how Fallout 3 holds up (this isn't everything, just the ones I have some knowledge of).
You're not comparing apples to apples, you need to be looking at other western cRPG games that came out around the same time if you're going to honestly compare writing, as the writing of an on rails FPS is very different than the writing requirements of an RPG.
And just a few months before 2008 you had
Neverwinter Nights 2: Mask of the Betrayer... which from a writing perspective blows Fallout 3 (and 4... and pretty much every Bethesda title) out of the water as well as the original
Mass Effect (which has inferior writing to NWN2:MotB, but definitely superior writing to FO3). In early 2009 you also had
Divinity II - Ego Draconis which having played both FO3 and it I think Divinity II has a better written story, if weaker open world and replayablity than FO3. And while it released late 2009 and thus I feel is not really comparable to FO3, you also saw
Dragon Age: Origins... which... yeah, has writing up there with NWN2:MotB.
And in 2008 you had
Fable II which... honestly, I remember less of the story of that than I do of FO3, so it definitely had weaker writing.
There's a few Diablo clones thrown into the RPG category, but I have trouble counting them as their gameplay design makes writing less of a focus. Oh, and I left off JRPGs too because while they are comparable, they're also very different and more linear and so again the writing isn't as comparable.
So if we restrict ourselves to 2008, FO3's writing does seem to hold up... but it was also released in a drought year for western cRPGs. 2007 and 2009 both had titles that had writing that simply was near legendary tier (both Mask of the Betrayer and Dragon Age: Origins were held up as having writing on nearly on par with Balder's Gate 2 and Planescape: Torment, two of the best written western cRPGs of all time). As such, for the RPG genre as a whole, FO3 managed to come out in a year where it didn't have stiff competition in the writing department, which helps it hold up, but a quick look at the year before and after reveal that it wasn't some standout as far as the genre goes.