Ok, heres how it works in BW.
As far as scourge goes they weren't really a threat to carrs since tosses would almost always have ample amount of sairs before making carrs.(in fact, zergs made devs to counter sairs and in turn carrs are brought in.)
The strat starts off with sair/reaver into carrs eliminating any scourge counter and dweb > dark swarm.(or the air units were fast enough to dodge swarm and rape another base if so inclined).
Carrs were mainly used against T anyways. (in which case, I usually didn't even need support aside from obs unless he made a huge amount of wraiths) But since Tempests are weaker against air, air(or anti-air) would certainly be needed as support.
If Phoenixes are any better than Sairs, then I can't see how scourge would help here, either. Then again, I'm just speculating at this point since the game isn't out, yet.
Carrs were considered to be too good a unit(even on its own)especially vs T. That's why I think they're being weakened.
Not sure why you mention valks, though. I hope you're not saying valks > carrs since thats laughable. Wraiths,yes, valks, no.