I gotta be honest here and say that I would be really, really nervous about going back and tweaking the older bags, particularly when it comes to something like the RBP and such. The workaround with the Final Respite bag forced me to create a "new" version of the bag that ends up being spawned to your inventory in the tradeskill process - This new version has a new resource ID and a new HASH code, making it a separate entry in the database from the older version.
The issue here comes in terms of things like tradeskill checks, or even checks in missions and unique requirements, and so on and so forth; to make these bags actually have packs, they would have to receive new versions in the database, and these new versions by default wouldn't work with any pre-existing tradeskills or anything of the sort; being frank, I'm afraid that in messing around with these older items, sooner or later there'd be something that we'd miss, and it'd end up causing a massive headache for both us and for the playerbase.
I'm not saying that it isn't a *possibility* down the line, but at the moment trying to track down all potential issues is something that would really take a lot more time than it's worth right now; we've got a pretty full dance card these days, and while I agree that giving these items pack functionality would be *nice* I also can't sit here and say that, with everything that'd have to be checked and everything that could potentially go wrong, it's a 'high-priority' kinda issue.
Now that we've thought of a work-around solution, though, it may be something we end up doing at a later date, perhaps even in the itemization round of The Rebalancing work. It's just... not quite as easy as it sounds.
Edit: As a note, looking through a few things, we may actually be fixing this particular bug via code in the near future, which would allow backpacks looted from a corpse to actually maintain their pack functionality. No promises there, but if so it'd save us from having to deal with going back and making alterations to any older backpacks in the game (which would be pretty awesome). =)