Let's go over the facts.
1) UR releases new New Blood packs
that are somewhat more cost-efficient, however, they cost more credits so that they will take longer for free players to amass.
2) UR has 50-60k players a week. The overwhelming majority of those do not buy credits at all, or buy them very rarely. So it follows that the vast majority of UR's population hasn't opened ANY new NB packs
3) Nobody has bothered to figure out exactly what the long-term economic impact of these packs
will be. It's too complicated. Most likely the fact that they have a better rare yield will be offset partially or completely by the fact that free players take longer to get them. Predicting that they will have a huge impact on the economy is, therefore, absurd.
4) Lots of cards went down in price. There are several possible explanations for this. However, the new NB packs
have not been around for a long time, and as I said above, would not have been accessible to most players. It is EXTREMELY UNLIKELY that despite this, the new NB packs
have already created a huge influx of rares on the market. Maybe they introduced A LITTLE BIT MORE than the old NB packs
5) People notice cards are falling in price, freak out, blame new NB packs
without any proof. Where is the proof? That's right, there isn't any.
In light of these facts, yes, it makes sense to hold on to those cards.