I think this is going to mean a lot more trades, and I've been listening to MLB radio this morning and they feel the same. To be honest it could mean Arrieta gets traded this offseason but I think that would depend on being able to trade for an Archer or the like.
My thought was basically that you probably get 70% of whatever Archer costs for Arrieta. I think they are largely equivalent talents on the field. The last 2 years Archer's been worth 8.3 fWAR to Arrieta's 11.1 fWAR. Arreita is 3 years older but given he's a health nut I think he'll age decently well. The main difference is the years of control. If that assumption is accurate, I think it presents some intriguing opportunities. First of all, Arrieta easily becomes a "cheaper" alternative to Sale/Archer types. Where as teams might want multiple top 50 prospects for Archer/Sale, you might be able to get Arrieta for two top 100 guys one being a top 50. In that respect, he basically under cuts the market on those two but they prop up his value higher more than it might ordinarily be.
I think the cubs are going to be in fairly hard on Archer if he's moved just because the years of control mainly. I'm not going to start another debate on his value but if Arrieta is 70% of Archer's value then you would essentially be talking about trading Arrieta + 30% for Archer which would seem like a win in the cubs book because instead of paying whatever Archer costs you're paying 30% of it. And the additional side effect here is that it presents them a chance to swap out misfitting parts. For example, you probably would be trading several bats that don't fit as well in the cubs system. Conversely, you might be able to to deal Arrieta to someone like Houston since I'm doubting they want to part with George Springer or Alex Bregman to acquire Sale/Archer. On the other hand, they probably would move guys like Francis Martes and David Paulino as the head line pieces to get Arrieta. You might basically be swapping Archer and Arrieta and with the prospect side you might be trading 2-3 good young hitters for 2 good young pitchers.
Even if you ended up with a net loss of talent, you might actual come away with more usable talent.