That's an exaggeration. It's only not worth 100 gold if somebody has every GvG card they could ever want and has stopped buying GvG packs altogether. This is a really small portion of the population. Hell I'm sitting on over 10k dust and I still open GvG packs from time to time because I want every card in the set and I don't have them all yet. I'm only throwing about 100 gold into the next expansion piggy bank each week.
I don't think this makes sense. You're assuming people will buy GvG packs for 100 gold each until they have every GvG card they could ever want (this seems obviously untrue). You're assuming the present value of GvG packs that someone might purchase months from now is still 100 gold. You're ignoring that someone who plays a lot of Arena plans to get many more GvG packs without spending 100 gold specifically to purchase them.
Like, consider an alternative reward to 2 packs and some gold which is 1 pack and some gold plus 100. Do you really think that right after getting the latter reward most players would go buy one more pack than they'd otherwise have bought? If not, what's going on, in your view, if it's not that they'd rather have the gold than the pack?
I suppose I'm thinking of "most players" as "most players who get 10-12 wins in Arena" and not most players period. Certainly most of the entire playerbase is often happy to exchange 100 gold for either kind of pack and does so just about any time they have the gold to spare.