No, it is replacing the RSN package, but with Diamond Sports having financial issues, the RSN package was already going to collapse. They had already sold back everything other than football, men's basketball, and regular season women's basketball last season. ESPN probably would have been able to fit in the remaining football, but some of the men's basketball and probably all of the women's basketball would have just been streaming on ACCNX/ESPN+.
The CW is the 5th most available channel (maybe the 6th behind PBS, but that's non-commercial so it's not really a factor) and way above any pay channel. They have historically had fewer hours of programming than the four networks above them so their local affiliates are used to having lots of time to fill however they want and controlling those commercials, which is why there are limits on how much they are going to want to purchase. In addition to being on pay TV, it is available OTA, which a lot of households have returned to while dropping pay TV. RSNs have been the first thing that the so called skinny bundles, popular on internet-based TV, but also increasingly an option on traditional pay TV, have not bothered adding or dropped after their initial contracts expired, which is why Diamond Sports is having the issues in the first place. The RSN package was national in the sense that any of the RSNs in areas no covered by Diamond Sports channels were given the option to buy the rights and anyone outside of the conference footprint could watch on ESPN streaming.