Co2 in the way of 12g's i think are in for the long haul but i can see refillable Co2 tanks going the way of the dodoo. I do see a feild up here though that primarily offers Co2 as they dont have a HPA compressor as of yet.
Myself some of my gear REQUIRES HPA and thuse i just switched to HPA for everything (with the exception of obviously my PGP and T8.1 pistols). But in the switch i found plenty of good options in smaller light HPA tanks for when i playing pump (aka most of my time playing). If i playing reg play were i can fill offten nothing beats the light and tight package of my 13ci3000psi tank it feels very similare size wise to a 9 or 12 oz Co2 tank but only offers maybe 150 shots. For bigger out door games were i have the potential to be shooting bit more i usually opt for my Ninja 35ci3000psi steely good for around 450 rnds threw my SL68-II (not the most efficiant pump). And for big scenerio games i break out my Empire Ultra 56ci4500 Cabonfiber tank this is a very TIGHT setup thats still light but offers more shots than i can go threw with a pump (dont know exactly how many).
If ya feilds offer easy access to HPA and you play mostly at feilds (not outlaw balling) and your shooting markers that run fine on HPA (not old brass or tippys that REQUIRE Co2) i see no reasone why one would want to shoot Co2 over HPA. A 13ci3000psi tank is cheap and PERFECT for pump offers acceptable shot count especially for pumps and is no heavier or bigger than a 9 or 12oz Co2 tank.
All that being said i have in my collection 5 HPA tanks but still ALWAYS have a 12ox co2 tank with me as well just in case the Compressors go down and alll thats still available is Co2.