No, but one hell of a lot of them are. It would depend on what sorts of contracts and deals the two companies currently have.
Also, CBS and ABC are the only two remaining major network news radio operations. All the others are bit players with far fewer resources, and it shows in the quality and quantity of their newscasts. CC can't just switch every station they own to ABC, or to the lesser-quality offerings from NBC, Fox, et al (many of which don't even offer 24 hour service). This gives CBS leverage.
You're thinking old media. CBS is old media. Think new media. If CBS implodes, one of the second rate news providers will seize the opportunity.